Thermoelectric power generating systems utilizing segmented thermoelectric elements

ABSTRACT

A thermoelectric system includes a first thermoelectric element including a first plurality of segments in electrical communication with one another. The thermoelectric system further includes a second thermoelectric element including a second plurality of segments in electrical communication with one another. The thermoelectric system further includes a heat transfer device including at least a first portion and a second portion. The first portion is sandwiched between the first thermoelectric element and the second thermoelectric element. The second portion projects away from the first portion and configured to be in thermal communication with a working medium.

CONTINUING APPLICATION DATA

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/136,334, filed May 24, 2005 and incorporated in its entiretyby reference herein, which is a continuation of U.S. Pat. No. 6,959,555,filed Aug. 18, 2003 and incorporated in its entirety by referenceherein, which is a continuation-in-part of U.S. Pat. No. 7,231,772,filed Aug. 23, 2002 and incorporated in its entirety by referenceherein, and which is a continuation-in-part of U.S. Pat. No. 7,111,465,filed Mar. 31, 2003 and incorporated in its entirety by referenceherein, which is a continuation of U.S. Pat. No. 6,539,725, filed Apr.27, 2001 and incorporated in its entirety by reference herein, which isrelated to and claims the benefit of U.S. Provisional Patent ApplicationNo. 60/267,657 filed Feb. 9, 2001 and incorporated in its entirety byreference herein. This application also claims the benefit of U.S.Provisional Patent Application No. 60/834,006, filed Jul. 28, 2006,which is incorporated in its entireties by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This disclosure relates to improved configurations for solid-statecooling, heating and power generation systems.

2. Description of the Related Art

Thermoelectric devices (TEs) utilize the properties of certain materialsto develop a temperature gradient across the material in the presence ofcurrent flow. Conventional thermoelectric devices utilize P-type andN-type semiconductors as the thermoelectric material within the device.These are physically and electrically configured in such a manner thatthe desired function of heating or cooling is obtained.

The most common configuration used in thermoelectric devices today isillustrated in FIG. 1A. Generally, P-type and N-type thermoelectricelements 102 are arrayed in a rectangular assembly 100 between twosubstrates 104. A current, I, passes through both element types. Theelements are connected in series via copper shunts 106 saddled to theends of the elements 102. A DC voltage 108, when applied, creates atemperature gradient across the TE elements. TEs are commonly used tocool liquids, gases and solid objects.

Solid-state cooling, heating and power generation (SSCHP) systems havebeen in use since the 1960's for military and aerospace instrumentation,temperature control and power generation applications. Commercial usagehas been limited because such systems have been too costly for thefunction performed, and have low power density so SSCHP systems arelarger, more costly, less efficient and heavier than has beencommercially acceptable.

Recent material improvements offer the promise of increased efficiencyand power densities up to one hundred times those of present systems.However, Thermoelectric (TE) device usage has been limited by lowefficiency, low power density and high cost.

It is well-known from TE design guides (Melcor Corporation“Thermoelectric Handbook” 1995 pp. 16-17) that in today's TE materials,the cooling power at peak efficiency produced by a module with ZT=0.9 isabout 22% of the maximum cooling power. Thus, to achieve the highestpossible efficiency, several TE modules are required compared to thenumber required for operation at maximum cooling. As a result, the costof TE modules for efficient operation is significantly higher and theresulting systems are substantially larger.

It is known from the literature (for example, see Goldsmid, H. J.“Electronic Refrigeration” 1986, p. 9) that the maximum thermal coolingpower can be written as:

$\begin{matrix}{{q_{COPT} = {{I_{OPT}\alpha_{C}} - {\frac{1}{2}I_{OPT}^{2}R} - {K\; \Delta \; T}}},} & (1)\end{matrix}$

where:

q_(COPT) is the optimum cooling thermal power;

I_(OPT) is the optimum current;

α is the Seebeck Coefficient;

R is the system electrical resistance;

K is the system thermal conductance;

ΔT is the difference between the hot and cold side temperatures; and

T_(C) is the cold side temperature.

Further, from Goldsmid's:

$\begin{matrix}{{I_{OPT} = {{\frac{\alpha}{R}\frac{1}{( \sqrt{{ZT}_{AVE} - 1} )}} = \frac{\alpha}{R( {M - 1} )}}},} & (2)\end{matrix}$

where:

Z is the material thermoelectric figure of merit;

T_(AVE) is the average of the hot and cold side temperatures; and

SUBSTITUTION Equation (2) into (1) yields:

$\begin{matrix}{q_{OPT} = {\lbrack {{\frac{{ZT}_{C}}{( {M - 1} )}( {\frac{\Delta \; T}{T_{C}} - \frac{1}{2( {M - 1} )}} )} - {\Delta \; T}} \rbrack {K.}}} & (3)\end{matrix}$

The term on the right side of Equation (3) in brackets is independent ofthe size (or dimensions) of the TE system, and so the amount of coolingq_(OPT) is only a function of material properties and K For the geometryof FIG. 1, K can be written as:

$\begin{matrix}{{K = \frac{\lambda \; A_{C}}{L_{C}}},} & (4)\end{matrix}$

where λ is the average thermal conductivity of the N & P materials;A_(C) is the area of the elements; and L is the length of each element.

Since α is an intrinsic material property, as long as the ratio

Lc/Ac

is fixed, the optimum thermal power q_(OPT), will be the same. Forcurrent equal to I_(OPT), the resistance is:

$\begin{matrix}{{R_{C} = {{R_{OC} + R_{PC}} = {\frac{\rho_{TE}L_{C}}{A_{C}} + R_{PC}}}},} & (5)\end{matrix}$

where ρhd TE is the intrinsic average resistivity of the TE elements;R_(OC) is the TE material resistance; and R_(PC) is parasiticresistances.

For the moment, assume R_(P) is zero, then R is constant. I_(OPT) isconstant if

L_(C)/A_(C)

is fixed. Only if the ratio

Lc/Ac

changes, does K and hence, q_(COPT) and R_(OC) and hence, I_(OPT)changes.

Generally, it is advantageous to make a device smaller for the samecooling output. An important limitation in thermoelectric systems isthat as, for example, the length L_(C) is decreased for fixed A_(C), theratio of the parasitic resistive losses to TE material losses, φ_(C)becomes relatively large:

$\begin{matrix}{\varphi_{C} = {\frac{R_{PC}}{R_{OC}}.}} & (6)\end{matrix}$

This can be seen by referring to FIG. 1C, which depicts a typical TEcouple. While several parasitic losses occur, one of the largest for awell-designed TE is that from shunt 106. The resistance of shunt 106 perTE element 102 is approximately,

$\begin{matrix}{{R_{PC} \approx {( \frac{B_{C} + G_{C}}{W_{C}T_{C}} )P_{SC}}},} & (7)\end{matrix}$

where G_(C) is the gap between the TE elements; B_(C) is the TE elementand shunt breadth; W_(C) is the TE element and shunt width; T_(C) is theshunt thickness; and P_(SC) is the shunt resistivity.

For the geometry of FIG. 1, the resistance for a TE element is:

$\begin{matrix}{{R_{OC} = \frac{P_{TE}L_{C}}{B_{C}W_{C}}},} & (8)\end{matrix}$

where L_(c) is the TE element length.Thus, using Equations (7) and (8) in (6):

$\begin{matrix}{\varphi_{C} \approx {{B_{C}( \frac{B_{C} + G_{C}}{T_{C}L_{C}} )}{( \frac{P_{SC}}{P_{TE}} ).}}} & (9)\end{matrix}$

SUMMARY OF THE INVENTION

In certain embodiments, a thermoelectric system is provided. Thethermoelectric system comprises a first thermoelectric elementcomprising a first plurality of segments in electrical communicationwith one another. The thermoelectric system further comprises a secondthermoelectric element comprising a second plurality of segments inelectrical communication with one another. The thermoelectric systemfurther comprises a heat transfer device comprising at least a firstportion and a second portion. The first portion is sandwiched betweenthe first thermoelectric element and the second thermoelectric element.The second portion projects away from the first portion and configuredto be in thermal communication with a working medium.

In certain embodiments, a thermoelectric system is provided. Thethermoelectric system comprises a plurality of thermoelectric elements,at least some of the thermoelectric elements comprising a plurality ofsegments. The thermoelectric system further comprises a plurality ofheat transfer devices, at least some of the heat transfer devicescomprising at least a first portion and a second portion. The firstportion is sandwiched between at least two thermoelectric elements ofthe plurality of thermoelectric elements so as to form at least onestack of thermoelectric elements and heat transfer devices. The secondportion projects away from the stack and configured to be in thermalcommunication with a working medium.

In certain embodiments, a method of fabricating a thermoelectric systemis provided. The method comprises providing a plurality ofthermoelectric elements, at least some of the thermoelectric elementscomprising a plurality of segments. The method further comprisesproviding a plurality of heat transfer devices, at least some of theheat transfer devices comprising at least a first portion and a secondportion. The method further comprises assembling the plurality ofthermoelectric elements and the plurality of heat transfer devices toform at least one stack of alternating thermoelectric elements and heattransfer devices. The first portions of the heat transfer devices aresandwiched between at least two neighboring thermoelectric elements. Thesecond portions of the heat transfer devices project away from the stackand configured to be in thermal communication with a working medium.

These and other aspects of the disclosure will be apparent from thefigures and the following more detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-1B depicts a conventional TE module.

FIG. 1C depicts a conventional TE couple.

FIG. 2 depicts a general arrangement of a SSCHP system with thermalisolation and counter flow movement of its working media.

FIG. 3 depicts the temperature changes that occur in the media, as theworking media progress through the system.

FIGS. 4A-4B depict a system with three TE modules, four fin heatexchangers, and liquid-working media.

FIGS. 5A-5B depict a system with two TE modules, a segmented heatexchanger to achieve a degree of thermal isolation with a single heatexchanger, and counter flow of the liquid media.

FIG. 6 depicts and gaseous media system with two TE modules and ductedfans to control fluid flow.

FIGS. 7A-7D depict a solid media system with counter flow to furtherenhance performance. The TE elements utilize a high length to thicknessratio to achieve added thermal isolation.

FIG. 8 depicts a system with TE elements arranged so that current passesdirectly through the array and thereby lowers cost, weight and sizewhile providing improved performance.

FIG. 9 depicts a system with TE elements, heat pipes and heat exchangersthat is simple and low cost. The hot side and cold side are separated bythermal transport through heat pipes.

FIG. 10 depicts a fluid system in which the fluid is pumped through theheat exchanger and TE module array so as to achieve a low temperature atone end to condense moisture out of a gas or a precipitate from a liquidor gas. The system has provisions to shunt working fluid flow to improveefficiency by lowering the temperature differential across portions ofthe array.

FIG. 11 depicts an array in which working fluid enters and exits at avariety of locations, and in which part of the system operates incounter flow and part in parallel flow modes.

FIG. 12 depicts a stack TE system with reduced parasitic electricalresistive losses.

FIG. 13A depicts details of a TE element and heat exchange member in apreferred embodiment for a stack system.

FIG. 13B depicts a section of a stack system constructed from elementsshown in FIG. 13A.

FIG. 14 depicts another TE element and heat exchanger configuration.

FIG. 15 depicts yet another TE element and heat exchanger configuration.

FIG. 16 depicts a stack configuration with two vertical rows of TEelements electrically in parallel.

FIG. 17 depicts a cooling/heating assembly with two rows of TE elementselectrically in parallel.

FIG. 18 depicts another configuration with two TE elements electricallyin parallel.

FIG. 19 depicts a heat exchanger element with one portion electricallyisolated from another portion.

FIG. 20 depicts another configuration of a heat exchanger element withone portion electrically isolated from another portion.

FIG. 21 depicts yet another configuration of a heat exchanger with oneportion electrically isolated from another portion.

FIG. 22 depicts a heat exchanger segment configured in an array ofelectrically and thermally isolated portions.

FIG. 23 depicts a cooler/heater constructed in accordance with theconcepts of FIG. 22.

FIG. 24A depicts a heat exchange segment with TE elements aligned in thedirection of fluid flow.

FIG. 24B depicts segments of FIG. 24A configured as an isolated elementheat exchanger array in which electrical current flows generallyparallel to working medium flow.

FIG. 25A depicts segments of a design configured as an isolated elementheat exchanger array in which electrical current flows generallyperpendicular to the direction of current flow.

FIG. 25B depicts a plan view of the assembly in FIG. 25A.

FIG. 26A depicts a TE heat exchanger module with reduced parasiticelectrical resistance, which operates at relatively high voltage.

FIG. 26B depicts a plan view of a heat exchanger array that uses TEmodules of FIG. 26A.

FIG. 27 depicts an isolated element and stack configuration with heattransfer to moving solid members.

FIG. 28 depicts an isolated element stack array with heat transferbetween a liquid and a gas.

FIG. 29 depicts a heat exchanger module with low parasitic electricalresistance for use in the stack array of FIG. 28.

FIG. 30 depicts a segment of an isolated element heat exchanger withsolid heat sink and moving gaseous working fluid.

FIG. 31A depicts a heat exchanger element with TE elements generally inthe center to about double heat transfer from the element.

FIG. 31B depicts another heat transfer element generally for liquidswith the TE element generally in the center.

FIG. 31C depicts yet another heat exchanger with the TE elementgenerally in the center.

FIG. 32 schematically illustrates a portion of an example thermoelectricsystem in accordance with certain embodiments described herein.

FIGS. 33A and 33B show the figures of merit (ZT) as functions oftemperature for various P-type and N-type thermoelectric materials,respectively, compatible with certain embodiments described herein.

FIG. 34 depicts the figure of merit, ZT, as a function of temperaturefor three different compositions of lead telluride doped with variouslevels of iodine.

FIG. 35 shows the power curve compatibility conflict among three TEelements constructed in series in the direction of flow.

FIG. 36 shows the power curves among three TE elements with varyingaspect ratios in accordance with certain embodiments described herein.

FIG. 37 schematically depicts a pair of segmented TE elements in aconventional configuration.

FIG. 38 shows the average efficiencies for three differentconfigurations simulated using a model calculation.

FIG. 39 shows an example of a model analysis of a thermoelectric systemwhere the parameter being varied is the TE thickness.

FIG. 40 shows an example prototype system built using six Bi₂Te₃ TEelements sandwiched between seven copper heat transfer devices.

FIG. 41 is a graph showing power generation curves for the sixindividual Bi₂Te₃ elements of FIG. 40.

FIG. 42 shows experimental results for initial testing of segmented TEelements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the context of this description, the terms thermoelectric module andTE module are used in the broad sense of their ordinary and accustomedmeaning, which is (1) conventional thermoelectric modules, such as thoseproduced by Hi Z Technologies, Inc. of San Diego, Calif., (2) quantumtunneling converters, (3) thermionic modules, (4) magneto caloricmodules, (5), elements utilizing one, or any combination ofthermoelectric, magneto caloric, quantum, tunneling and thermioniceffects, (6) any combination, array, assembly and other structure of (1)through (6) above. The term thermoelectric element, is more specific toindicate an individual element that operates using thermoelectric,thermionic, quantum, tunneling, and any combination of these effects.

In the following descriptions, thermoelectric or SSCHP systems aredescribed by way of example. Nevertheless, it is intended that suchtechnology and descriptions encompass all SSCHP systems.

Accordingly, the invention is introduced by using examples in particularembodiments for descriptive and illustrative purposes. A variety ofexamples described below illustrate various configurations and may beemployed to achieve the desired improvements. In accordance with thepresent description, the particular embodiments and examples are onlyillustrative and not intended in any way to restrict the inventionspresented. In addition, it should be understood that the terms coolingside, heating side, cold side, hot side, cooler side and hotter side andthe like, do not indicate any particular temperature, but are relativeterms. For example, the “hot,” side of a thermoelectric element or arrayor module may be at ambient temperature with the “cold,” side at acooler temperature than the ambient. The converse may also be true.Thus, the terms are relative to each other to indicate that one side ofthe thermoelectric is at a higher or lower temperature than thecounter-designated temperature side.

Efficiency gains for geometries described in U.S. Pat. No. 6,539,735,entitled Improved Efficiency Thermoelectrics Utilizing ThermalIsolation, yield an additional 50% to 100% improvement for manyimportant applications. Combined with the material improvements beingmade, system efficiency gains of a factor of four or more appearpossible in the near future. The prospects of these substantialimprovements have lead to renewed interest in the technology and theeffort to develop SSCHP systems for new applications.

In general, this disclosure describes a new family of SSCHPconfigurations. These configurations achieve compact, high-efficiencyenergy conversion and can be relatively low cost. Generally, severalembodiments are disclosed wherein TE elements or modules (collectivelycalled elements in this text) are sandwiched between heat exchangers.The TE elements are advantageously oriented such that for any twoelements sandwiching a heat exchanger, the same temperature type sidefaces the heat exchanger. For example, the cooler side of each of the TEelements sandwiching a heat exchanger face the same heat exchanger orshunt, and thus each other. In a group of configurations, at least oneworking medium is passed sequentially through at least two heatexchangers so that the cooling or heating provided is additive on theworking medium. This configuration has the added benefit that itutilizes the advantages of thermal isolation, as described in U.S. Pat.No. 6,539,725, in manufacturable systems that exhibit high systemefficiency and power density as noted in the references above. Asexplained in that patent, in general, a TE device achieves increased orimproved efficiency by subdividing the overall assembly of TE elementsinto thermally isolated subassemblies or sections. For example, the heatexchangers may be subdivided so as to provide thermal isolation in thedirection of working medium flow. For example, a TE system has aplurality of TE elements forming a TE array with a cooling side and aheating side, wherein the plurality of TE elements are substantiallyisolated from each other in at least one direction across the array.Preferably, the thermal isolation is in the direction of the workingmedia flow. This thermal isolation can be provided by having a heatexchanger configured in sections such that the heat exchanger hasportions which are thermally isolated in the direction of working fluidflow.

In the present disclosure, having sequential use of heat exchangers ofthe same temperature type for the working fluid provides a type ofthermal isolation in itself. In addition, the heat exchangers or the TEelements, or TE modules or any combination may be configured to providethermal isolation in the direction of the working fluid flow over andabove the thermal isolation provided by having a series or sequence ofheat exchangers through which at least one working fluid passes insequence.

The principles disclosed for cooling and/or heating applications, areequally applicable to power generation applications, and anyconfiguration, design detail, and analogous part that may be combined inany way to produce an assembly for power generation, is also applicable.The system may be tuned in a manner to maximize the efficiency for thegiven application, but the general principles apply.

The embodiments described in this application lower the constructioncomplexity and cost of SSCHP devices while still maintaining orimproving efficiency gains from thermal isolation.

Also disclosed are several embodiments for reducing cost by using lessTE material and facilitating operation closer to peak efficiency. Manyembodiments achieve a substantial reduction in parasitic losses (see,e.g., FIGS. 12-31).

One aspect of the disclosed embodiments involves a thermoelectric systemhaving a plurality of N-type thermoelectric elements and a plurality ofP-type thermoelectric elements. Preferably, a plurality of first shuntsand a plurality of second shunts are provided. At least some of thefirst shunts are sandwiched between at least one N-type thermoelectricelement and at least one P-type thermoelectric element, and at leastsome of the second shunts sandwiched between at least one P-typethermoelectric element and at least one N-type thermoelectric elements,so as to form a stack of thermoelectric elements, with alternating firstand second shunts, wherein at least some of the first shunts and atleast some of the second shunts project away from the stack in differingdirections.

Preferably, the thermoelectric elements are constructed to be quitethin, such as from 5 microns, to 1.2 mm, from 20 microns to 200 micronsfor superlattice and heterostructure thermoelectric designs, and inanother embodiment from 100 to 600 microns. These designs provide forsignificant reduction in the usage of thermoelectric material.

In one embodiment, the thermoelectric system further comprises a currentsource electrically coupled to the stack, the drive current traversingthrough the heat transfer devices and thermoelectric elements in series.In another embodiment, the heat transfer devices thermally isolate atleast some of the P-type thermoelectric elements from at least some ofthe N-type thermoelectric elements.

In one embodiment, the heat transfer devices accept a working fluid toflow through them in a defined direction. Preferably, the heat transferdevices are heat exchangers and may have a housing with one or more heatexchanger elements inside.

In another embodiment, at least some of the first shunts are constructedof a first electrode portion electrically isolated from and thermallycoupled to a second shunt portion.

FIG. 2 illustrates a first generalized embodiment of an advantageousarrangement for a thermoelectric array 200. The array 200 has aplurality of TE modules 201, 211, 212, 213, 218 in good thermalcommunication with a plurality of first side heat exchangers 202, 203,205 and a plurality of second side heat exchangers 206, 207 209. Thedesignation first side heat exchanger and second side heat exchangerdoes not implicate or suggest that the heat exchangers are on one sideor the other side of the entire SSCHP system, but merely that they arein thermal communication with either the colder side or the hotter sideof the thermoelectric modules. This is apparent from the figure in thatthe heat exchangers are actually sandwiched between thermoelectricmodules. In that sense, they are in thermal communication with a firstside or a second side of the thermoelectric modules. The colder side ofa first TE module 201 is in thermal contact with a first side heatexchanger 205 and the hot side of the TE module 201 is in thermalcontact with an inlet second side heat exchanger 206. A second workingmedia 215, such as a fluid, enters the array 200 in the upper right handcorner of FIG. 2 through the inlet second side heat exchange 206, andexits near the lower left from a final or outlet second side heatexchanger 209. A first working media 216 enters at the upper leftthrough an inlet first side heat exchanger 202 and exits near the lowerright from a final or outlet first side heat exchanger 205. Electricalwires 210 (and similarly for other TE Modules) connected to a powersupply, not shown, connect to each TE module 201. First conduits 208,represented as lines on FIG. 2, convey the second working media 215 andsecond conduits 204 convey the first working media 216 sequentiallythrough various heat exchangers 202, 203, 205, 206, 207 and 209 asdepicted.

In operation, the second working media 215 absorbs heat from the TEmodule 201 as it passes downward through the inlet second side heatexchanger 206. The second working media 215 passes through conduit 208and upwards into and through the second side heat exchanger 207. In goodthermal communication with the heat exchanger 207 are the hotter sidesof the TE modules 211 and 212, which have been configured so that theirrespective hotter sides face toward one another to sandwich the secondside heat exchanger 207. The second side working media 215, is furtherheated as it passes through the second side heat exchanger 207. Thesecond side working media 215 next passes through the second side heatexchanger 209, where again, the hotter sides of the TE modules 213 and218 sandwich and transfer heat to the second side heat exchanger 209,further heating the second side working media 215. From the heatexchanger 209, the second working media 215 exits the array 200 from theoutlet or final second side heat exchange 209.

Similarly, the first working media 216 enters the inlet first side heatexchanger 202 at the upper left corner of FIG. 2. This heat exchanger202 is in good thermal communication with the colder side of the TEmodule 218. The first working media 216 is cooled as it passes throughthe inlet first side heat exchanger 202, on through another first sideexchanger 203 and finally through the outlet first side heat exchanger205, where it exits as colder working media 217.

The thermoelectric cooling and heating is provided by electrical powerthrough wiring 210 into TE module 218, and similarly into all the otherTE modules.

Thus, in sum, working media is placed in good thermal contact with thecold side of the TE module at the left hand side of the array, so thatheat is extracted from the media. The media then contacts a second andthird TE module where additional heat is extracted, further cooling themedia. The process of incremental cooling continues, as the mediaprogresses to the right through the desired number of stages. The mediaexits at the right, after being cooled the appropriate amount.Concurrently, a second media enters the system at the far right and isincrementally heated as it passes through the first stage. It thenenters the next stage where it is further heated, and so on. The heatinput at a stage is the resultant of the heat extracted from theadjacent TE modules' cold sides, and the electrical power into thosemodules. The hot side media is progressively heated as it moves in ageneral right to left direction.

In addition to the geometry described above, the system provides benefitif both media enter at the same temperature and progressively get hotterand colder. Similarly, the media can be removed from or added to thecool or hot side at any location within the array. The arrays can be ofany useful number of segments such as 5, 7, 35, 64 and larger numbers ofsegments.

The system can also be operated by reversing the process with hot andcold media in contact with TE modules, and with the hot and cold mediamoving from opposite ends (as in FIG. 2 but with the hot media enteringas media 216 and the cold media entering as media 215). The temperaturegradient so induced across the TE modules produces an electric currentand voltage, thus converting thermal power to electrical power. All ofthese modes of operation and those described in the text that followsare part of the inventions.

As illustrated in FIG. 2, the separation of the heat exchanger into asequence of stages provides thermal isolation in the direction of flowof the working media from TE module to TE module. U.S. Pat. No.6,539,725, entitled First Improved Efficiency Thermoelectrics UtilizingThermal Isolation, filed Apr. 27, 2001 describes in detail theprinciples of thermal isolation which are exhibited throughout thisdescription with various specific and practical examples for easymanufacturing. This patent application is hereby incorporated byreference in its entirety.

As described in U.S. Pat. No. 6,539,725, the progressive heating andcooling of media in a counter flow configuration as described in FIG. 2,can produce higher thermodynamic efficiency than under the sameconditions in a single TE module without the benefit of the thermalisolation. The configuration shown in FIG. 2, thus presents an SSCHPsystem 200 that obtains thermal isolation through the segments or stagesof heat exchangers sandwiched between thermoelectric modules in acompact easily producible design.

In addition to the features mentioned above, the thermoelectric modulesthemselves may be constructed to provide thermal isolation in thedirection of media flow and each heat exchanger or some of the heatexchangers may be configured to provide thermal isolation in aindividual heat exchanger through a configuration as will be describedfurther in FIG. 5 or other appropriate configurations. In general, theheat exchanger could be segmented in the direction of flow to provideincreased thermal isolation along the flow of a single TE module such asthe TE module 218 and the inlet heat exchanger 202.

FIG. 3 depicts an array 300 of the same general design as in FIG. 2,consisting of a plurality of TE modules 301 and colder side heatexchangers 302, 305, and 307 connected so that a first working medium315 follows the sequential heat exchanger to heat exchanger path shown.Similarly, a plurality of hot side heat exchangers 309, 311 and 313convey a hotter side working medium 317 in a sequential or staged mannerin the direction shown by the arrows. The TE modules 301 are arrangedand electrically powered as in the description of FIG. 2.

The lower half of FIG. 3 depicts the cold side temperatures ortemperature changes 303, 304, 306, 308 of the colder side working mediumand hot side temperatures 310, 312, 314 of the hotter side workingmedium.

The colder side working medium 315 enters and passes through an inletcolder side heat exchanger 302. The working medium's temperature drop303 in passing through the inlet colder side heat exchanger 302 isindicated by the drop 303 in the cold side temperature curve Tc. Thecolder side working medium 315 is further cooled as it passes throughthe next stage colder side heat exchanger 305, as indicated by atemperature drop 304 and again as it passes through a third colder sideheat exchanger 307, with an accompanying temperature drop 306. Thecolder side working medium 315 exits as colder fluids 316 at temperature308. Similarly, the hotter side working medium 317 enters a first orinlet hotter side heat exchanger 309 and exits at a first temperature310 as indicated by the hotter side temperature curve T_(H) in the FIG.3. The hotter side working medium progresses through the array 300 instages as noted in FIG. 2, getting progressively hotter, finally exitingafter passing through outlet hotter side heat exchanger 313 as hotterworking fluid at 318 and at a hotter temperature 314. It is readily seenthat by increasing the number of stages (that is TE modules and heatexchangers) the amount of cooling and heating power can be increased,the temperature change produced by each heat exchanger can be reduced,and/or the amount of media passing through the array increased. Astaught in the U.S. Pat. No. 6,539,725, efficiency also can increase withmore stages, albeit at a diminishing rate.

Experiments and the descriptions referenced above, show that thermalisolation and the progressive heating and cooling achievable with theconfiguration of FIGS. 2 and 3 can result in significant efficiencygains, and are therefore important. With such systems, gains of over100% have been achieved in laboratory tests.

FIG. 4A depicts an array 400 with three TE modules 402, four heatexchangers 403 and two conduits 405 configured as described in FIGS. 2and 3. Colder and hotter side working fluids enters at a colder sideinlet 404 and a hotter side inlet 407, respectively and exitrespectively at a colder side exit 406 and a hotter side exit 408. FIG.4B is a more detailed view of one embodiment of a heat exchanger 403. Itis shown as a type suitable for fluid media. The heat exchanger assembly403, has consists of an outer housing 412 with an inlet 410 and an exit411, heat exchanger fins 414, and fluid distribution manifolds 413. Theoperation of array 400 is essentially the same as described in FIGS. 2and 3. The number of the TE modules 402 is three in FIG. 4, but could beany number. Advantageously, the housing 412 is thermally conductive,being made from a suitable material such as corrosion protected copperor aluminum. In one embodiment, heat exchanger fins 414 advantageouslyare folded copper, or aluminum soldered or braised to the housing 412,so as to achieve good thermal conductivity across the interface to theTE Module. The Fins 414 can be of any form, but preferably of a designwell suited to achieve the heat transfer properties desired for thesystem. Detailed design guidelines can be found in “Compact HeatExchangers”, Third Edition by W. M. Kays and A. L. London.Alternatively, any other suitable heat exchangers can be used, such asperforated fins, parallel plates, louvered fins, wire mesh and the like.Such configurations are known to the art, and can be used in any of theconfigurations in any of FIGS. 2 through 11.

FIG. 5A depicts an alternative configuration to that of FIG. 4 for theconduit connections to provide flow from heat exchanger stage to heatexchanger. The array 500 has first and second TE modules 501 and 510,three heat exchangers 502, 503 and 506, and a conduit 504. Of course, aswith previous embodiments and configurations, the particular number oftwo first side heat exchangers 502, 503 and one second side heatexchanger 506 is not restrictive and other numbers could be provided.

FIG. 5B illustrates an enlarged view of a preferred embodiment for theheat exchangers 502, 503, 506. This heat exchanger configuration asshown in FIG. 5B would be appropriate for the other embodiments and canbe used in any of the configuration in FIGS. 2-8 and FIG. 11. Thisadvantageous embodiment for one or more of the heat exchangers in suchconfigurations has an outer housing 516 with segmented heat exchangerfins 511 separated by gaps 513. Working fluid enters through an inlet505 and exits through exit 508. As an alternative to gaps, the heatexchanger could be made so that it is anisotropic such that it isthermally conductive for a section and non-thermally conductive foranother section rather than having actual physical gaps between heatexchanger fins. The point is for thermal isolation to be obtainedbetween stages of an individual heat exchanger segment and anotherindividual heat exchanger segment in the direction of flow. This wouldbe thermal isolation provided in addition to the thermal isolationprovided by having stages of heat exchangers in the embodimentsdescribed in FIGS. 2-5.

Advantageously, a first working fluid 507 which, for example is to beheated, enters an inlet 505 and passes downward through an inlet orfirst heat exchanger 502 in thermal communication with a first TE module501. The working fluid 507 exits at the bottom and is conducted tosubsequent heat exchanger 503 through conduit 504, where it again passesin a downward direction past a second TE module 510 and exits through asa hotter working 508. Preferably, a second working fluid 517 enters fromthe bottom of FIG. 5A through inlet 518 and travels upward through athird heat exchanger 506 past the colder sides (in the present example)of TE modules 501 and 510. The heat exchanger 506 is in good thermalcommunication with the colder sides of the TE modules 501 and 510. Bythis arrangement, the working fluids 507 and 517 form a counter flowsystem in accordance with the teaching of U.S. Pat. No. 6,539,725referenced above.

Preferably, the heat exchangers 502, 503 and 506, shown in detail inFIG. 5B, are constructed to have high thermal conductivity from thefaces of the TE modules 501, 510, 510, through the housing 516 and tothe heat exchanger fins 511 (depicted in four isolated segments).However, it is desirable to have low thermal conductivity in thedirection of flow, so as to thermally isolate each heat exchangersegment from the others. If the isolation is significant, and TE modules501 and 510 do not exhibit high internal thermal conductivity in theirvertical direction (direction of working fluid flow), the array 500benefits from the thermal isolation and can operate at higherefficiency. In effect, the array 500 can respond as if it were an arrayconstructed of more TE Modules and more heat exchangers.

FIG. 6 depicts yet another heater/cooler system 600 that is designed tooperate beneficially with working gases. The heater/cooler 600 has TEmodules 601, 602 in good thermal communication with first side heatexchangers 603, 605 and second side heat exchangers 604. A first workingfluid, such as air or other gases 606, is contained by ducts 607, 708,610 and a second working fluid 616 is contained by ducts 615, 613. Fansor pumps 609, 614 are mounted within ducts 608, 615.

The first working fluid 606 enters the system 600 through an inlet duct607. The working fluid 606 passes through a first heat exchanger 603where, for example, it is heated (or cooled). The working fluid 606 thenpasses through the fan 609 which acts to pump the working fluid 606through the duct 608, and through the second heat exchanger 605, whereit is further heated (or cooled), and out an exit duct 610. Similarly, aworking fluid, such as air or another gas, enters through an inlet duct615. It is pushed by a second fan or pump 614 through a third heatexchanger 604 where, in this example, it is cooled (or heated). Thecooled (or heated) working fluid 616 exits through an exit duct 613.

The system 600 can have multiple segments consisting of additional TEmodules and heat exchangers and isolated, segmented heat exchangers asdescribed in FIG. 5B. It can also have multiple fans or pumps to provideadditional pumping force. In addition, one duct, for example 607, 608,can have one fluid and the other duct 613, 615 a second type of gas.Alternately, one side may have a liquid working fluid and the other agas. Thus, the system is not restricted to whether a working medium is afluid or a liquid. Additionally, it should be noted that the exit duct613 could be routed around the fan duct 609.

FIG. 7A depicts a heating and cooling system 700 for beneficial use witha fluid. The assembly has a plurality of TE modules 701 with a pluralityof first side working media 703 and a plurality of second side workingmedia 704. In the present example, both the first side working media 703and the second side working media 704 form disks. The first side workingmedia 703 are attached to a first side shaft 709, and the second sideworking media 704 are attached to a second side shaft 708. The shafts708, 709 are in turn attached to first side motor 706 and second sidemotor 705, respectively, and to corresponding bearings 707. Thepreferred direction of motor rotation is indicated by arrows 710 and711.

A separator 717 both divides the array into two portions and positionsthe TE modules 701. The TE modules 701, held in position by theseparator 717, are spaced so as to alternately sandwich a first sideworking medium 703 and a second side working medium 704. For any two TEmodules 701, the modules are oriented such that their cold sides and hotsides face each other as in the previous embodiments. The working media703, 704 are in good thermal communication with the TE elements 701.Thermal grease or the like is advantageously provided at the interfacebetween the thermoelectric element 701 and the working media 703, 704.The purpose of the grease becomes apparent in the discussion belowregarding the operation of the working media 703, 704. A first sidehousing section 714 and second side housing section 715 contain fluidconditioned by the system 700. Electrical wires 712, 713 connect to theTE modules 701 to provide drive current for the TE modules.

FIG. 7B is a cross sectional view 7B-7B through a portion of the system700 of FIG. 7A. A first fluid 721 and a second fluid 723 are representedalong with their direction of flow by arrows 721 and 723. The firstfluid exits as represented by the arrow 722 and a second exits asrepresented by the arrow 724. The system 700 operates by passing currentthrough electrical wires 712 and 713 to TE modules 701. The TE modules701 have their cold and hot sides facing each other, arranged in thefashion as described in FIGS. 2 and 3. For example, their adjacent coldsides both face the first side working media 703 and their hot sidesface the second side working media 704. The Separator 717 serves thedual function of positioning the TE modules 701 and separating the hotside from the cooled side of the array 700.

For an understanding of operation, assume, for example, that a secondfluid 723 is to be cooled. The cooling occurs by thermal exchange withsecond side media 704. As the second side media 704 rotate, the portionof their surface in contact with the colder side of the TE modules 701at any given time is cooled. As that portion rotates away from the TEmodules 701 through the action of the second motor 705, the second media704 cool the second side fluid that then exits at exit 724. The secondfluid is confined within the array 700 by the housing section 715 andthe separator 717.

Similarly, the first fluid 721 is heated by the first side media 703 inthermal contact with the hotter side of the TE modules 701. Rotation(indicated by arrow 711) moves the heated portion of first media 703 towhere the first fluid 721 can pass through them and be heated viathermal contact. The first fluid 721 is contained between the housing714 and the separator 717 and exits at exit 722.

As mentioned above, thermally conductive grease or liquid metal such asmercury, can be used to provide good thermal contact between the TEmodules 701 and the media 703, 704 at the region of contact.

As mentioned above, the configuration of FIGS. 7A and 7B may also beadvantageously used to cool or heat external components such asmicroprocessors, laser diodes and the like. In such instances, the diskswould contact the part using the thermal grease or liquid metal or thelike to transfer the heat to or from the part.

FIG. 7C depicts a modified version of the system 700 in which the TEmodules 701 are segmented to achieve thermal isolation. FIG. 7C shows adetailed view of the portion of array 700 in which TE modules 701 and702 transfer thermal power to heat moving media 704 and 703 (therotating discs in this example). The moving media 704 and 703 rotateabout axes 733 and 734, respectively.

In one embodiment, advantageously, the working media 704 and 703 rotatein opposite directions as indicated by arrows 710 and 711. As movingmedia 704, 703 rotate, heat transfer from different sections of TEmodules 701 and 702 come into thermal contact with them andincrementally change the temperature of the moving media 704, 703. Forexample, a first TE module 726 heats moving medium 704 at a particularlocation. The material of the moving media 704 at that location movesinto contact with a second TE module 725 as moving medium 704 rotatescounter clockwise. The same portion of moving medium 704 then moves onto additional TE module segments 701. The opposite action occurs asmoving medium 703 rotates counterclockwise and engages TE modules 701and then subsequently TE modules 725 and 726.

Advantageously, moving media 704, 703 have good thermal conductivity inthe radial and axial directions, and poor thermal conductivity in theirangular direction, that is, the direction of motion. With thischaracteristic, the heat transfer from one TE module 725 to another TEmodule 726 by conductivity through the moving media 704 and 708 isminimized, thereby achieving effective thermal isolation.

As an alternative to TE modules or segments 701, 725, 726, a single TEelement or several TE element segments may be substituted. In this case,if the TE elements 701 are very thin compared to their length in thedirection of motion of moving media 704, 703, and have relatively poorthermal conductivity in that direction, they will exhibit effectivethermal isolation over their length. They will conduct heat and thusrespond thermally as if they were constructed of separate TE modules701. This characteristic in combination with low thermal conductivity inthe direction of motion within the moving media 704, 703 can achieveeffective thermal isolation and thereby provides performanceenhancements.

FIG. 7D depicts an alternative configuration for moving media 704, 703in which the media are constructed in the shape of wheels 729 and 732with spokes 727 and 731. In the spaces between spokes 727 and 731 and ingood thermal contact with them, are heat exchanger material 728 and 730.

The system 700 can operate in yet another mode that is depicted in FIG.7D. In this configuration, working fluid (not shown) moves axially alongthe axes of the array 700 passing through moving media 704, 703sequentially from one medium 704 to the next moving medium 704, and soon in an axial direction until it passes through the last medium 704 andexits. Similarly, a separate working fluid, not shown, passes throughindividual moving medium 703 axially through array 700. In thisconfiguration, the ducts 714 and 715 and separator 717 are shaped toform a continuous ring surrounding moving media 704, 703 and separatingmedium 704 from medium 703.

As the working fluid flows axially, thermal power is transferred to theworking fluid through heat exchanger material 728 and 730.Advantageously, the hot side working fluid, for example, passes throughheat exchanger 728, moves through the array 700 in the oppositedirection of the working fluid moving through heat exchanger 730. Inthis mode of operation, the array 700 acts as a counterflow heatexchanger, and a succession of sequential heat exchangers 728 and 730incrementally heat and cool the respective working fluids that passthrough them. As described for FIG. 7C, the thermally active componentscan be TE modules 701 that can be constructed so as to have effectivethermal isolation in the direction of motion of the moving media 704,703. Alternatively, the TE modules 701 and 702 can be segments asdescribed in FIG. 7C. In the latter case, it is further advantageous forthe thermal conductivity of the moving media 704, 703 to be low in thedirection of motion so as to thermally isolate portions of the outerdiscs 729 and 732 of the moving media 704, 703.

Alternately, the design could be further contain radial slots (notshown) in the sections 729 and 732 that are subject to heat transferfrom TE modules 701 and 702 to achieve thermal isolation in thedirection of motion.

FIG. 8 depicts another embodiment of a thermoelectric system an 800having a plurality of TE elements 801 (hatched) and 802 (unhatched)between first side heat exchangers 803 and second side heat exchangers808. A power supply 805 provides current 804 and is connected to heatexchangers 808 via wires 806, 807. The system 800 has conduits and pumpsor fans (not shown) to move hot and cold side working media through thearray 800 as described, for example, in FIGS. 2, 3, 4, 5, 6 and 7.

In this design, the TE modules (having many TE elements) are replaced byTE elements 801 and 802. For example, hatched TE elements 801 may beN-type TE elements and unhatched TE elements 802 may be P-type TEelements. For this design, it is advantageous to configure heatexchangers 803 and 808 so that they have very high electricalconductivity. For example, the housing of the heat exchangers 803, 808and their internal fins or other types of heat exchanger members can bemade of copper or other highly thermal and electrical conductivematerial. Alternately, the heat exchangers 803 and 808 can be in verygood thermal communication with the TE elements 801 and 802, butelectrically isolated. In which case, electrical shunts (not shown) canbe connected to the faces of TE elements 801 and 802 to electricallyconnect them in a fashion similar to that shown in FIG. 1, but with theshunts looped past heat exchangers 803 and 808.

Regardless of the configuration, DC current 804 passing from N-type 801to P-type TE elements 802 will, for example, cool the first side heatexchanger 803 sandwiched between them, and current 804 passing fromP-type TE elements 802 to N-type TE elements 801 will then heat thesecond side heat exchanger 808 sandwiched between them.

The Array 800 can exhibit minimal size and thermal losses since theshunts, substrates and multiple electric connector wires of standard TEmodules can be eliminated or reduced. Further, TE elements 801 and 802can be heterostructures that accommodate high currents if the componentsare designed to have high electrical conductivity and capacity. In sucha configuration, the array 800 can produce high thermal power densities.

FIG. 9 depicts a thermoelectric system 900 of the same general type asdescribed in FIG. 8, with P-type TE elements 901 and N-type TE elements902 between, and in good thermal contact with first side heat transfermembers 903 and second side heat transfer members 905. In thisconfiguration, the heat transfer members 903 and 905 have the form ofthermally conductive rods or heat pipes. Attached to, and in goodthermal communication with the heat transfer members 903 and 905 areheat exchanger fins 904, 906, or the like. A first conduit 907 confinesthe flow of a first working medium 908 and 909 and a second conduit 914confines the flow of a second working fluid 910 and 911. Electricalconnectors 912 and 913 conduct current to the stack of alternatingP-type and N-type TE elements 901, 902, as described in FIG. 8.

In operation, by way of example, current enters the array 900 throughthe first connector 912, passes through the alternating P-type TEelements 901 (hatched) and N-type TE elements 902 (unhatched) and exitsthrough the second electrical connector 913. In the process, the firstworking media 908 becomes progressively hotter as it is heated byconduction from heat transfer fins 904, which in turn have been heatedby conduction through the first heat transfer members 903. The firstconduit 907 surrounds and confines a first working media 908 so it exitsat a changed temperature as working fluid 909. Portions of the firstconduit 907 thermally insulate the TE elements 901 and 902 and thesecond side heat transfer members 905 from the first (hot in this case)working media 908 and 909. Similarly, the second working media 910enters through the second conduit 914, is cooled (in this example) as itpasses through the second side heat exchangers 906 and exits as cooledfluid 911. The TE elements 901, 902 provide cooling to the second sideheat transfer members 905 and hence, to heat exchanger fins 906. Thesecond side conduit 914 acts to confine the second (cooled in thisexample) working media 910, and to insulate it from other parts of array900.

Although described for individual TE elements in the embodiments ofFIGS. 8-9, TE modules may be substituted for the TE elements 901, 902.In addition, in certain circumstances, it may be advantageous toelectrically isolate TE elements 901, 902 from the heat transfer members903, 905, and pass current through shunts (not shown). Also, the heatexchangers 904, 906 can be of any design that is advantageous to thefunction of the system. As with the other embodiments, it is seen thatthe configurations of FIGS. 8 and 9 provide a relatively easilymanufacturable system that also provides enhanced efficiency fromthermal isolation. For example, in FIG. 8, the heat exchangers 808, 803which alternate between P-type and N-type thermal electric elements,will either be of the colder or hotter heat exchanger type, but will bereasonably thermally isolated from each other and cause thethermoelectric elements of the P and N type to be reasonably thermallyisolated from one another.

FIG. 10 depicts another thermoelectric array system (1000) that providesthermal isolation. Advantageously, this configuration may perform thefunction of a system that utilizes cooling and heating of the samemedium to dehumidify, or remove precipitates, mist, condensable vapors,reaction products and the like and return the medium to somewhat aboveits original temperature.

The system 1000 consists of a stack of alternating P-type TE elements1001 and N-type TE elements 1002 with interspersed cold side heattransfer elements 1003 and hot side heat transfer elements 1004. In thedepicted embodiment, heat exchanger fins 1005, 1006 are provided forboth the colder side heat transfer elements 1003 and the hotter sideheat transfer elements 1004. A colder side conduit 1018 and a hotterside conduit 1019 direct working fluid 1007, 1008 and 1009 within thearray 1000. A fan 1010 pulls the working fluid 1007, 1008 and 1009through the array 1000. Preferably, colder side insulation 1012thermally isolates the working fluid 1007 while travelling through thecolder side from the TE element stack and hotter side insulation 1020preferably isolates the working fluid while travelling through thehotter side from the TE element stack. A baffle 1010 or the likeseparates the colder and hotter sides. In one preferred embodiment, thebaffle 1010 has passages 1010 for working fluids 1021 to pass through.Similarly, in one embodiment, fluid passages 1017 allow fluid 1016 toenter the hot side flow passage.

A screen 1011 or other porous working fluid flow restrictor separatesthe colder from the hotter side of array 1000. Condensate, solidprecipitate, liquids and the like 1013 accumulate at the bottom of thearray 1000, and can pass through a valve 1014 and out a spout 1015.

Current flow (not shown) through TE elements 1001 and 1002, cools colderside heat transfer elements 1003 and heats hotter side heat transferelements 1004, as discussed in the description of FIG. 9. In operation,as the working fluid 1007 passes down the colder side, precipitate,moisture or other condensate 1013 from the working fluid 1007 cancollect at the bottom of the array 1000. As required, the valve 1014 canbe opened and the precipitate, moisture or condensate 1013 can beremoved through the spout 1015 or extracted by any other suitable means.

Advantageously, some of the working fluid 1021 can be passed from thecolder to the hotter side through bypass passages 1020. With thisdesign, not all of the colder side fluid 1007 passes through the flowrestrictor 1011, but instead can be used to reduce locally thetemperature of the hotter side working fluid, and thereby improve thethermodynamic efficiency of the array 1000 under some circumstances.Proper proportioning of flow between bypass passages 1020 and flowrestrictor 1011, is achieved by suitable design of the flow propertiesof the system. For example, valves can be incorporated to control flowand specific passages can be opened or shut off. In some uses, the flowrestrictor 1011 may also act as a filter to remove precipitates fromliquid or gaseous working fluids 1008, or mist or fog from gaseousworking fluids 1008.

Advantageously, additional hotter side coolant 1016 can enter array 1000through side passages 1017, also for the purpose of reducing the hotterside working fluid temperature or increasing array 1000 efficiency.

This configuration can produce very cold conditions at the flowrestrictor 1011, so that working fluid 1008 can have substantial amountsof precipitate, condensate or moisture removal capability. In analternative mode of operation, power to the fan 1010 can be reversed andthe system operated so as to heat the working fluid and return it to acool state. This can be advantageous for removing reaction products,precipitates, condensates, moisture and the like that is formed by theheating process. In one advantageous embodiment, flow restrictor 1011,and/or heat exchangers 1005 and 1006 can have catalytic properties toenhance, modify, enable, prevent or otherwise affect processes thatcould occur in the system. For liquid working fluids, one or more pumpscan replace fan/motor 1010 to achieve advantageous performance.

FIG. 11 depicts a thermoelectric array 1100 similar in design to that ofFIGS. 2 and 3, but in which working media has alternate paths throughthe system. The array 1100 has TE modules 1101 interdispersed betweenheat exchangers 1102. A plurality of inlet ports 1103, 1105 and 1107conduct working media through the array 1100. A plurality of exit ports1104, 1106 and 1108 conduct working media from the array 1100.

In operation, by way of example, working media to be cooled enters at afirst inlet port 1103 and passes through several of the heat exchangers1102, thereby progressively cooling (in this example), and exits througha first exit port 1104. A portion of the working media that removes heatfrom array 1100 enters through a second inlet port 1105, passes throughheat exchangers 1102, is progressively heated in the process, and exitsthrough a second exit port 1106.

A second portion of working media to remove heat enters a third inletport 1107, is heated as it passes through some of the heat exchangers1102 and exits through a third exit port 1108.

This design allows the cool side working media which passes from thefirst inlet port 1103 to the first exit port 1104 to be efficientlycooled, since the hot side working media enters at two locations in thisexample, and the resultant temperature differential across the TEmodules 1101 can be on average lower than if working media entered at asingle port. If the average temperature gradient is lower on average,then under most circumstances, the resultant system efficiency will behigher. The relative flow rates through the second and third inlet port1105 and 1107 can be adjusted to achieve desired performance or torespond to changing external conditions. By way of example, higher flowrates through the third inlet port 1107, and most effectively, areversal of the direction of flow through that portion so that thirdexit port 1108 is the inlet, can produce colder outlet temperatures inthe cold side working media that exits at first exit port 1104.

The basic underlying connections for a conventional thermoelectric 100are shown in additional detail in FIG. 1C. As mentioned above, a P-typeelement 110 and an N-type element 112 are of the type well known to theart. Shunts 106 are attached to, and in good electrical connection with,P-type and N-type TE elements 110 and 112. Generally, large numbers ofsuch TE elements and shunts are connected together to form a TE module,as shown in FIG. 1A.

The length of TE elements 110, 112 in the direction of current flow isL_(C) 116; their breadth is B_(C) 117; their width is W_(C) 118, andtheir distance apart is G_(C) 120. The thickness of shunts 106 is T_(C)109.

The dimensions B_(C), W_(C), and L_(C), along with the TE material'sfigure of merit, Z, the current 122 and the operating temperaturesdetermine the amount of cooling, heating or electrical power produced,as is well known to the art (See Angrist, S. W. “Direct EnergyConversion” 3^(rd) Ed. 1977 Ch. 4, for example).

The design depicted in FIG. 12 alters the conventional construction ofFIG. 1 in a manner to reduce the amount of thermoelectric materialrequired, and the magnitude of the parasitic resistance in the shunts106. A TE configuration 1200 has a plurality of first side TE elements1201, 1202 of alternating conductivity types sandwiched in seriesbetween shunts 1203 and a plurality of second side shunts 1204, so thata current 1209 passes perpendicular to the breadth B_(B) and width W_(B)of the shunts rather than generally parallel to the breadth as in FIG.1C. For the design of FIG. 12, the ratio, φ_(B) of R_(PB) to R_(OB) is:

$\begin{matrix}{\varphi_{B} \approx \frac{R_{PB}}{R_{OB}}} & (10)\end{matrix}$

Where;

$\begin{matrix}{R_{PB}\frac{P_{SB}T_{B}}{B_{B}W_{B}}} & (11) \\{R_{OB} = \frac{P_{TE}L_{B}}{B_{B}W_{B}}} & (12)\end{matrix}$

so,

$\begin{matrix}{\varphi_{B} \approx {( \frac{T_{B}}{B_{B}} )( \frac{P_{SB}}{P_{TE}} )}} & (13)\end{matrix}$

Where

T_(B) is the shunt thickness

L_(B) is the TE element length

ρ_(SB) is the shunt resistivity

B_(B) is the TE element and shunt active breadth

W_(B) is the TE elements and shunt active width

If φ_(C) is set equal to φ_(B), then the parasitic electrical resistancelosses will have the same proportional effect on the performance of theconfigurations of FIG. 1C and FIG. 12. For comparative purposes, assumematerial properties of the two configurations are identical, then;

φ_(C)=φ_(B)  (14)

or using Equations (9 and 12) in B;

$\begin{matrix}{\frac{L_{C}}{L_{B}} \approx {B_{C}( \frac{B_{C} + G_{C}}{T_{C}T_{B}} )}} & (15)\end{matrix}$

For today's typical thermoelectric modules;

B_(C)≈1.6 mm.

W_(C)≈1.6 mm.

G_(C)≈1.6 mm.

T_(C)≈0.4 mm.

and assume;

T_(B)≈2 mm.

P_(SB)≈P_(SC)

then,

$\begin{matrix}{\frac{L_{c}}{L_{B}} \approx 6.4} & (16)\end{matrix}$

Thus the length L_(B) can be

1/6.4

that of L_(C) and the resulting resistive losses of the design in FIG.12 do not exceed those of a conventional TE module. If this is the case,and all other losses are negligible or decrease proportionally, a TEsystem utilizing the configuration of FIG. 12 would have the sameoperating efficiency as that of FIG. 1C, but with

L_(B) = L_(C)/6.4  .

The volume of the new configuration can be compared to that of FIG. 1C.For the same q_(OPT), the area ratio must remain the same, so;

$\begin{matrix}{\frac{L_{B}}{A_{B}} = \frac{L_{C}}{A_{C}}} & (17)\end{matrix}$

and since;

$\begin{matrix}{\frac{L_{B}}{L_{C}} = \frac{1}{6.4}} & (18) \\{A_{C} = {6.4\mspace{11mu} {A_{B}.}}} & (19)\end{matrix}$

The volume ratio of thermoelectric material of the two is;

V_(C)=A_(C)L_(C)  (20)

V_(B)=A_(B)L_(B)  (21)

and;

$\begin{matrix}{\frac{V_{B}}{V_{C}} = {( \frac{A_{B}}{A_{C}} )( \frac{L_{B}}{L_{C}} )}} & (22) \\{\approx \frac{1}{6.4^{2}} \approx {1/41}} & (23)\end{matrix}$

Therefore with these assumptions,

1/41

as much TE material is required. This substantial potential reduction,while it may not be fully realized because of the exactitude ofassumptions made, nevertheless can be very beneficial in reducing theamount of TE material used and hence, cost and size as well.

The TE stack configuration 1200 of FIG. 12 has P-type TE elements 1201and N-type TE elements 1202 of length L_(B) 1205. The direction ofcurrent flow is indicated by the arrow 1209. The TE elements have abreadth B_(B) and a width W_(B). Between P-type TE elements 1201 andN-type TE elements 1202, in the direction of current flow, are thesecond side shunts 1204 (“PN shunts”). Between N-type 1202 and P-type1201 elements, in the direction of current flow, are the first sideshunts 1203 (“NP shunts”). The PN shunts 1204 extend generally in theopposite direction from the stack 1200 than the NP shunts 1203. Anglesother than 180° are also advantageous.

If an appropriate current 1209 is passed in the direction indicated, NPshunts 1203 are cooled and PN shunts 1204 are heated. Through thisconfiguration, the parasitic electrical resistance losses for theconfiguration 1200 are lower typically than for the conventionalconfiguration 100 of FIG. 1 for the same TE element dimensions. Thus, ifthe TE length L_(B) 1205 is reduced to equate the ratio of parasiticelectrical losses in the two configurations, the TE length L_(B) 1205will be smaller, and the configuration of FIG. 12 advantageously canoperate at higher power density than that of FIG. 1. As a result, theconfiguration 1200 of FIG. 12 also uses less thermoelectric material,and can be more compact than in the conventional design of FIG. 1.

The shunts 1203, 1204 can serve the dual function of transmittingthermal power away from the TE elements 1201, 1202 and exchange thermalpower with an external object or medium, such as a working fluid.

An illustration of a preferred embodiment 1300 of a shunt combined toform a heat exchanger 1302 is depicted in FIG. 13A. Preferably, at leastone TE element 1301 is electrically connected, such as with solder, to araised electrode surface 1303 of a heat exchange shunt 1302.Advantageously, the shunt 1302 can be constructed primarily of a goodthermal conductor, such as aluminum, and have integral clad overlaymaterial 1304, 1305, made of a high-electrical conductivity material,such as copper, to facilitate TE element 1301 attachment and currentflow at low resistance.

FIG. 13B depicts a detailed side view of a portion of a stackthermoelectric assembly 1310 made up of the thermoelectric shunts 1302and TE elements 1301 of FIG. 13A. A plurality of shunts 1302 with raisedelectrode surfaces 1303 are electrically connected in series to TEelements 1301 of alternating conductivity types.

The shunts 1302 will be alternately heated and cooled when anappropriate current is applied. The thermal power produced istransported away from the TE elements 1301 by the shunts 1302.Advantageously, the raised electrodes 1303 facilitate reliable,low-cost, stable surfaces to which to attach the TE elements 1301. Inpractice, a stack of a plurality of these assemblies 1310 may beprovided. An array of stacks could also be used which also furtherfacilitates thermal isolation.

The electrodes 1303 advantageously can be shaped to prevent solder fromshorting out the TE elements 1301. Also, the electrodes 1303advantageously can be shaped to control the contact area and hence,current density, through the TE elements 1301.

An example of a portion of a shunt heat exchanger 1400 is depicted inFIG. 14. This portion 1400 has increased surface area to aid heattransfer. A TE element 1401 is attached to a shunt 1402, preferablyconstructed as depicted in FIG. 13A, or as in other embodiments in thisapplication. Heat exchangers 1403, 1404, such as fins, are attached withgood thermal contact, such as by brazing, to the shunt 1402. In thisembodiment, a working fluid 1405 passes through the heat exchangers1403, 1404.

Advantageously, the shunt portion 1400 is configured so that as theworking fluid 1405 passes through the heat exchangers 1403, 1404,thermal power is transferred efficiently. Further, the size of materialsand proportions of the shunt 1402 and heat exchangers 1403, 1404 aredesigned to optimize operating efficiency when combined into a stacksuch as described in FIGS. 12 and 13B. Advantageously, the heatexchangers 1403, 1404 can be louvered, porous or be replaced by anyother heat exchanger design that accomplishes the stated purposes suchas those described in “Compact Heat Exchangers”, Third Edition, by W. M.Kays and A. L. London. The heat exchangers 1403, 1404 can be attached tothe shunt 1402 by epoxy, solder, braze, weld or any other attachmentmethod that provides good thermal contact.

Another example of a shunt segment 1500 is depicted in FIG. 15. Theshunt segment 1500 is constructed of multiple shunt elements 1501, 1502,1503 and 1504. The shunt elements 1501, 1502, 1503 and 1504 may befolded over, brazed, riveted to each other or connected in any other waythat provides a low electrical resistance path for a current 1507 topass and to provide low thermal resistance from a TE element 1506 to theshunts 1501, 1502, 1503 and 1504. The TE element 1506 is advantageouslyattached to segment 1500 at or near a base portion 1505.

The shunt segment 1500 depicts a design alternative to the shunt segment1400 of FIG. 14, and can be configured in stacks as depicted in FIGS. 12and 13, and then in arrays of stacks if desired. Both the configurationsin FIGS. 14 and 15 can be automatically assembled to lower the laborcost of the TE systems made from these designs.

Shunt segments can also be formed into stack assemblies 1600 as depictedin FIG. 16. Center shunts 1602 have first side TE elements 1601 of thesame conductivity type at each end on a first side and second side TEelements 1605 of the opposite conductivity type at each end of theopposite side of the center shunts 1602. Between each center shunt 1602to form a stack of shunts 1602 is placed a right shunt 1603 and a leftshunt 1604, as depicted in FIG. 16. The right shunts 1603 are placedsuch that the left end is sandwiched between, the TE elements 1601, 1605in good thermal and electrical contact. Similarly, the left side shunts1604 are positioned such that the right end is sandwiched between TEelements 1601, 1605, and are in good thermal and electrical contact. Theshunts 1602, 1603 and 1604 are alternately stacked and electricallyconnected to form a shunt stack 1600. A first working fluid 1607 and asecond working fluid 1608 pass through the assembly 1600. Of course, forthe embodiments shown in FIG. 16 and of the stack configurationsdescribed herein, the stack may be, and likely will, consist of manyadditional shunt elements in the stack. The small portions of a stackassembly 1600 are merely depicted to provide the reader with anunderstanding. Further replication of such stacks is clear from thefigures. In addition, additional stacks, thermally isolated in adirection of working fluid flow could be provided.

When a suitable current is applied in the one direction through the TEelements 1601, shunts 1605, 1604, the center shunts 1602 will be cooledand the left and right shunts 1604 and 1606 will be heated. As a result,the first working fluid 1607 passing through the center shunts 1602 willbe cooled and the second working fluid 1608 passing through the rightand left shunts 1603, 1604 will be heated. The stack assembly 1600 formsa solid-state heat pump for conditioning fluids. It is important to notethat the stack 1600 can have few or many segments and can therebyoperate at different power levels, depending on the amount of currentand voltage applied, component dimensions and the number of segmentsincorporated into the assembly. Arrays of such stacks may also beadvantageous. In a situation where arrays of such stacks 1600 are used,it would be preferable to provide thermal isolation in the direction offluid flow as described in U.S. Pat. No. 6,539,725 for improvedefficiency.

It should also be understood that the shunts 1602, 1603, 1604 can bereplaced by other shapes such as, but not limited to, those depicted inFIGS. 14 and 15, to improve performance.

A variation to the stack assembly 1600 depicted in FIG. 16 isillustrated in FIG. 17. For this configuration, a TE assembly 1700 isconstructed of right side shunts 1703 and left side shunts 1704 to forma generally circular shape. The right side shunts 1703 areadvantageously configured to form a partial circle as are the left sideshunts 1704. In a preferred embodiment, the shunts which become coldduring operation may be either larger or smaller than the shunts thatbecome hot, depending on the particular goals of the device. It shouldbe noted that the substantially circular configuration is not necessary,and other configurations of the shunt segments shown in FIG. 17 tocreate a center flow portion could be used. For example, the right sideshunts could be half rectangles or half squares, and the left sideshunts 1704 could be half rectangles or squares. Similarly, one sidecould be multi-sided and one side could be arcuate. The particular shapeof the shunts are changeable. The TE elements 1701 and 1702, ofalternating conductivity type, as discussed for FIG. 16, areelectrically connected in series in the stack assembly 1700. Preferably,a fluid 1712 passes into the central region formed by the shunts 1703,1704. A first portion 1707 of the fluid 1712 passes between the rightside shunts 1703 and a second portion 1706 of the working fluid 1712passes between the left side shunts 1704. A power supply 1708 iselectrically connected to the TE elements by wires 1712, 1713 that areconnected to the stack at connections 1710 and 1711. A fan 1709 may beattached to one (or both) ends of the stack. A pump, blower, or the likecould be used as well.

When power is applied to the fan 1709, it pumps the working fluid 1712through the assembly 1700. When current is supplied with a polarity suchthat the right shunts 1703 are cooled, the first fluid portion 1707 ofworking fluid 1712 is cooled as it passes through them. Similarly, thesecond portion 1706 of working fluid is heated as it passes throughheated left side shunts 1704. The assembly 1700 forms a simple, compactcooler/heater with a capacity and overall size that can be adjusted bythe number of shunts 1703, 1704 utilized in its construction. It isapparent that the shunts 1703, 1704 could be angular, oval or of anyother advantageous shape. Further, the shunts can be of the designsdepicted in FIGS. 14, 15 or any other useful configuration.

In one embodiment of the thermoelectric system of FIGS. 12, 14, 15, 16and 17, more than one TE element can be used in one or more portions ofan array as is depicted in FIG. 18. In this example, TE elements 1801,1804 are connected to raised electrode surfaces 1804 on each side ofshunts 1802, 1803.

A number of TE elements 1801, electrically in parallel, can increasemechanical stability, better distribute thermal power and add electricalredundancy to the system. More than two TE elements 1801 can be used inparallel.

In certain applications, it is desirable to have exposed portions ofshunts in accordance with FIGS. 12-13 electrically isolated from anelectrode portion. One example of such a shunt is depicted in FIG. 19.In this embodiment, an electrical insulation 1905 isolates an electrodeportion 1903 of a shunt 1900 from a heat exchange portion 1904 of theshunt 1900. TE elements 1901, 1902 are preferably mounted on theelectrode portion 1903.

In operation, electrical potential is applied between TE elements 1901,1902 of opposite conductivity types, through, advantageously, theelectrode portion, 1903 made of a high electrical and thermalconductivity material, such as copper. Thermal power produced by the TEelements 1901, 1902 is conducted along the shunt electrode 1903, throughthe electrical insulation 1905, and into the heat exchange portion 1904of the shunt 1900. Advantageously, the electrical insulation 1905 is avery good thermal conductor such as alumina, thermally conductive epoxyor the like. As shown, the interface shape formed by electricalinsulation 1905 is a shallow “V” shape to minimize thermal resistance.Any other shape and material combination that has suitably lowinterfacial thermal resistance can be used as well. A stack of suchshunts 1900 can be used as described previously.

An alternate form of electrical isolation is shown in another shuntsegment 2000 assembly depicted in top view in FIG. 20. First TE elements2001 are connected to a left shunt 2003 of shunt segment array 2000, andsecond TE elements 2002 are connected to a right shunt 2004 of shuntsegment array 2000. Electrical insulation 2005 is positioned betweenleft side shunt segments 2003 and right side shunt segments 2004.

The configuration depicted in FIG. 20 provides electrical isolationbetween TE elements 2001 and 2002 while retaining mechanical integrityof the overall shunt 2000. In this configuration as drawn, theelectrical insulation 2005 need not provide particularly good thermalconductivity since the sources of thermal power, the TE elements 2001and 2002, can cool or heat the left and right shunt segments 2003, 2004,at different levels, provided electrical insulation 2005 is on averagecentered between the TE elements 2001 and 2002. It should be noted thatalthough two TE elements 2001 and two second TE elements 2002 aredepicted, a larger TE element or a larger number of TE elements on eachside could be utilized. Two first TE elements 2001 and two second TEelements 2002 are merely selected for illustration of a good stablemechanical structure. It should also be noted that depending on thedesired route for current, the first TE element 2001 and the second TEelements 2002 need not be, but may be, of differing conductivity types.

An alternate method of achieving electrical isolation within a shunt2100 is depicted in FIG. 21. A shunt portion 2103 with two first TEelements 2101 is mechanically attached to a second shunt portion 2104with two second TE elements 2102. Electrical insulation 2106mechanically attaches shunt portions 2103 and 2104, which are alsoseparated from one another by a gap 2105.

In cases where mechanical attachment 2106 is approximately centeredbetween the TE elements 2101 and 2102, and the TE elements 2101 and 2102produce about equal thermal power, the electrical insulation 2106 neednot be a good thermal conductor. The TE elements 2101 and 2102 eachprovide thermal power to the respective shunt portions 2103 and 2104.Electrical insulation 2106 can be adhesive-backed Kapton tape, injectionmolded plastic, hot melt adhesive or any other suitable material. Asshown in plan view in FIG. 21, the shunt portions 2103 2104 do notoverlap to form a lap joint. Such a joint, with epoxy or otherelectrically insulating bonding agent could also be used.

Another shunt segment array 2200, depicted in top view in FIG. 22, haselectrically isolated shunt segments in a rectangular TE array 2200.First TE elements 2201 are thermally connected to first shunt portions2202, and second TE elements 2203 are thermally connected to secondshunt portions 2204. Each shunt portion is separated electrically fromthe other shunt portions by gaps 2210, 2211. Electrical insulation 2208at the left side of the assembly, insulation 2207 in the middle andinsulation 2209 on the right side are preferably provided. An arrow 2212indicates working fluid flow direction. This configuration can beoperated at higher voltage and lower current than a similar arraywithout electrical isolation. As noted for FIG. 20, first TE elements2201 and second TE elements 2203 need not, but may be, of differingconductivity types. This will depend on the direction of desired currentflow. The TE elements 2202, 2203 may, however, be at differentpotentials.

The gaps 2210 serve to effectively thermally isolate first shuntportions 2202 from each other, and second shunt portions 2204 from eachother. Similarly, the side insulation 2208, 2209 provide both thermaland electrical isolation while mechanically attaching the shuntstogether. Center insulation 2207 provides electrical insulation andthermal isolation along its length. Thus, array 2200 is constructed toproduce thermal isolation in the direction of arrow 2212 as described inU.S. Pat. No. 6,539,725. This configuration can be operated at highervoltage and lower current than a similar array without electricalisolation.

A cooling system 2300 that employs shunt segment arrays generally of thetype described in FIG. 22, is depicted in FIG. 23. The cooling system2300 has inner shunt segments 2301, 2302 connected mechanically byelectrically insulating material 2320 such as tape. The inner shuntsegments 2302 are mechanically connected by electrically and thermallyinsulating material 2321. Similarly, the inner segments 2301 aremechanically connected by electrically and thermally insulating material2307. The inner shunt segments 2301, 2302 separately are connected to TEelements at the ends (not shown) in a manner described for FIG. 22. TheTEs are sandwiched in the stack between inner shunt segments 2301, 2302and respective outer shunt segments 2303, 2305. The center shuntsegments 2301 separately are connected to outer left shunt segments2305, and the inner shunt segments 2302 are connected to right outershunt segments 2303. Preferably, the right outer shunt segments 2303 aresimilarly mechanically connected by electrically and thermallyinsulating material 2322 which is similar to electrically insulatingmaterial 2321 connecting the inner shunt segments 2302. The left outershunt segments 2305 are similarly mechanically connected. A housing 2311holds a stack array of shunt segments and TEs. Terminal posts 2312 and2314 are electrically connected to inner segments 2301. Similarly,terminals 2315 and 2316 connect to inner shunt segments 2302.Preferably, thermally and electrically insulating spacers 2309, 2310 arepositioned between each inner and outer segment.

A first working fluid 2317 passes through the inner region and a secondworking fluid 2318, 2319 passes through the outer regions. When voltagesof the proper polarities and magnitude are applied between terminals2312 and 2314, 2315 and 2316, the inner shunt segments 2301, 2302 arecooled. Also, the outer shunt segments 2303, 2305 are heated. Thus, theworking fluid 2317 passing through the inner region is cooled, and theworking fluid 2318, 2319 passing through the outer shunt segments 2303,2305 is heated. The housing 2311 and the insulators 2309, 2310 containand separate the cooled fluid 2317 from the heated fluid 2318, 2319.

The electrical connections to energize each stack in the system 2300 canbe in series to operate at high voltage, in series/parallel to operateat about half the voltage or in parallel to operate at about ¼ thevoltage. Polarity could be reversed to heat the inner working fluid 2317and cool the outer working fluids 2318, 2319. More segments could beutilized in the direction of working fluids 2317, 2318, 2319 flow tooperate at even higher voltage and to achieve higher efficiency from theresultant more effective thermal isolation.

Another compact design that achieves enhanced performance from thermalisolation uses combined shunt and heat transfer segments 2400 asdepicted in FIGS. 24A and 24B. This design is very similar to that ofFIG. 14, but with TE elements 2401, 2402 aligned in the generaldirection of fluid flow. The TE elements 2401, 2402 of oppositeconductivity type are connected to an extension 2403 of a shunt 2404.Preferably, heat exchangers 2405, 2406, such as fins, are in goodthermal contact with the shunt 2404. A working fluid 2409 is heated orcooled as it passes through heat exchanger fins 2405, and 2406,depending on the direction of current flow.

FIG. 24B depicts a portion of a stack 2410 consisting of TE shuntsegments 2400 as shown in FIG. 24A. Current 2417 flows in the directionindicated by the arrow. A plurality of first side shunts 2400 and aplurality of second side shunts 2400 a are connected to TE elements2411. A first working fluid 2418 flows along the lower portion of stack2410 through the heat exchangers on the second side shunts 2400 a inFIG. 24 a, and a working fluid 2419 flows advantageously in the oppositedirection through the heat exchangers of first side shunts 2400.

When suitable current 2417 is applied, the upper portion of the stack2410 progressively cools fluid 2419 as it passes from one segment to thenext, and the lower portion progressively heats fluid 2418 as it passesfrom one shunt 2400 a to the next.

An alternative TE stack configuration 2500 is depicted in FIG. 25A. ThisTE stack achieves the benefits of thermal isolation with a working fluid2513 flowing generally perpendicular to the direction of current flow2512. A first shunt 2502 is connected electrically to a first TE element2501 and is in good thermal contact with heat exchangers 2503, 2504. Asecond first side shunt 2506 is similarly in good thermal contact withits heat exchangers 2508, and a third first side shunt 2505 is in goodthermal contact with its heat exchangers 2507. Interspersed between eachfirst side shunt 2502, 2506 and 2505 are TE elements 2501 of alternatingtype and second side shunts 2509, 2510 and 2511 projecting generally inthe opposite direction, as with FIG. 12. Second side shunts 2509, 2510and 2511, not fully depicted, are generally of the same shape and bearthe same spatial relationship as first side shunts 2502, 2506 and 2505.A working fluid 2513 passes through the stack assembly in the directionindicated by the arrow. When suitable current is applied verticallythrough the TE elements, first side shunts 2502, 2505 and 2506 areheated and second side shunts 2509, 2510 and 2511 are cooled. As theworking fluid 2513 passes first through heat exchanger 2507, thenthrough the heat exchanger 2508 and finally through the heat exchanger2503, it is progressively heated. A full stack assembly has repeatedsections of the array 2500, in the direction of current flow, assembledso that the top of heat exchanger 2503 would be spaced closely to thebottom of the next sequential heat exchanger 2504 of another arrayportion. The thermal isolation in the direction of working fluid 2513flow is readily apparent.

FIG. 25B is a plan view of the array portion 2500 depicted in FIG. 25A.The cooling of a plurality of TE elements 2501, alternating inconductivity type, are interspersed with the plurality of first sideshunts 2502, 2506, 2505, and a plurality of second side shunts 2511,2509 and 2510, so that the first side shunts 2502, 2506 and 2505alternate with the second side shunts 2511, 2509 and 2510. The shuntsare separated by gaps 2534 and are in good thermal contact with heatexchangers for each shunt. A first working fluid 2531 passes along theupper section from right to left and a working fluid 2532 passesadvantageously from left to right along the lower section. Thermal andelectrical insulation 2533 is preferably provided between each pair ofshunts, except where the electrical current flows through the TEs andshunts.

When suitable current passes through the array 2500, for example, theworking fluid 2531 is progressively heated and the working fluid 2532 isprogressively cooled. The insulation 2533 prevents unnecessary thermallosses and also prevents the working fluids 2531, 2532 from mixing. Thearray 2500, as shown, operates in counter flow mode, and employs thermalisolation to enhance performance. The same array 2500, can operate withthe working fluids 2531, 2532 moving in the same direction in parallelflow mode, and still have the benefits of thermal isolation to enhanceperformance. In either case, advantageously, the TE elements 2521 arenot all of the same resistance, but have resistances that vary dependingon the temperature and power differentials between individual TEelements, as described in U.S. Pat. No. 6,539,735.

Another TE module 2600 is depicted in FIG. 26A, that uses the principlesdiscussed in the present description to achieve operation at highervoltages and possible other benefits such as higher power density,compact size, ruggedness, higher efficiency. A first TE element 2601 issandwiched between a first end shunt 2603 and a second shunt 2604. Asecond TE element 2602, of opposite conductivity type is sandwichedbetween the second shunt 2604 and a third shunt 2605. This pattern iscontinued to final end shunt 2606. A current 2607 flows into final endshunt 2606, through the TE modules and out the first end shunt 2603, asindicated by arrows 2608 and 2609. Gaps 2611 prevent electricalconduction and reduce thermal conduction between adjacent shunts. In oneembodiment, the first end shunt 2603 and the final end shunt 2606 havean electrode surface 2612. The other shunts have shunt surfaces 2614that are thermally conductive but electrically insulating from the bodyof the shunts.

In operation, suitable current 2608 passes through the TE module 2600heating the upper surface and cooling the lower surface (or vice versa).The TE module 2600 depicted in FIG. 26A consists of five TE elements andsix shunts. Advantageously, any odd number of TE elements can beemployed, spaced alternately with shunts as depicted. Further, more thanone TE element (of the same type as explained for FIG. 18) may beconnected in parallel between each pair of shunts. To achievealternative functionality, an even number of TEs can be used, such as tohave electrical power confined to electrically isolated portions of onesurface.

An array 2620 of TE modules 2600 is depicted in FIG. 26B. FIG. 26Bdepicts two TE modules 2600, of the type shown in FIG. 26A, stacked ontop of each other with a center heat transfer member 2635 sandwichedbetween first side shunts 2604. Outer heat transfer members 2632 and2636 are thermally coupled to second side shunts 2605. The shunt andheat transfer members can also be of any other suitable types, forexample, the types presented in FIGS. 14 and 15. A first end shunt 2603of a first TE module is electrically connected to the outer heattransfer members 2632. Similarly, the other end shunt 2606 of the firstor upper TE module is electrically connected to the center heat transfermember 2635. Similarly, a second end shunt 2606 a of the second TEmodule is electrically coupled to the center heat transfer member 2635and the first end shunt 2603 a of the second TE module is electricallycoupled to the outer heat transfer member 2636 on the bottom of FIG.26B. Other than the end shunts, 2603, 2606, 2606 a and 2603 a, the othershunts 2604, 2605 have electrical insulation 2612 that is thermallyconductive. In addition, as in the arrangement of FIG. 26A, the shuntshave gaps 2611 to electrically isolate them from one another. Currentflow is indicated by the arrows 2628, 2629, 2630, 2631 and 2637. Asdepicted, the TE elements 2601, 2602 alternate in conductivity type.

When suitable current is passed through the array 2620, second sideshunts 2605 and the outer heat transfer members 2632 and 2636 areheated. The first side shunts 2604 and center heat transfer member 2635are cooled. The opposite is true for reversed current. The operatingcurrent can be adjusted along with the corresponding voltage byadjusting the dimensions and number of TE elements 2601, 2602.Similarly, power density can be adjusted. It should be noted that alarger number of shunts and TE elements could be used, which would widenthe configuration shown in FIG. 26B. In addition, further TE modules2600 could be stacked in a vertical direction. In addition, an array ofsuch stacks into or out of the plane of FIG. 26B could be provided orany combination of the above could be utilized. In a suitable array,thermal isolation principles in the direction of heat transfer orworking fluid flow could be utilized in accordance with the descriptionin U.S. Pat. No. 6,539,725.

An alternative embodiment of a TE module 2700, similar in type to the TEmodule 2600 of FIG. 26A, is illustrated in FIG. 27. End shunts 2705,2704 are electrically connected to a power source 2720 and ground 2709.TE elements 2701, 2702 are electrically connected to between the seriesof shunts 2703, 2704, 2705, 2706. In this embodiment, all shunts 2703,2704, 2705, 2706 are electrically isolated by insulation 2711 from firstand second heat transfer members 2707, 2708. The shunts are in goodthermal contact with the heat transfer members 2707, 2708. First sideheat transfer member 2708 moves in the direction indicated by an arrow2712. Advantageously, the second side heat transfer member 2707 moves inthe opposite direction, as indicated by an arrow 2710.

When suitable current is applied to the TE module 2700, the second sideheat transfer member 2707 is cooled and the first side heat transfermember 2708 is heated. Operation is similar to that associated with thedescription of FIGS. 7A, 7B, 7C, and 7D. It should be noted that thefirst and second heat transfer members 2707, 2708, need not berectangular in shape as might be inferred from FIG. 27, but may be diskshaped or any other advantageous shape, such as those discussed in FIG.7A. With effective design, the TE module 2700 can also achieve theperformance benefits associated with thermal isolation as discussed inU.S. Pat. No. 6,539,725.

In an alternative embodiment, heat transfer components 2707 and 2708 donot move. In that configuration, the TE module 2700 is similar to astandard module as depicted in FIG. 1, but can operate with a high powerdensity and utilize relative thin TE elements 2701, 2702.Advantageously, the TE module 2700 induces low shear stresses on the TEelements 2701, 2702 that are produced by thermal expansion differencesbetween the first side and second side shunts, for example. Becauseshear is generated in the TE module 2700 by the temperature differentialacross TE elements 2701, 2702, and is proportional to the widthdimension, it can be much less than the shear in a standard TE module,in which the shear is proportional to the overall module width. Thedifferences can be seen from a comparison of FIG. 12 with a standardmodule depicted in FIG. 1. Standard modules with more than two TEelements of the same dimensions as those in the configuration of FIG. 12will exhibit disadvantageously high shear stresses. Such stresses limitthermal cycling durability and module size.

FIG. 27 also provides a good illustration to describe how theembodiments described in this specification can be used for powergeneration as well. In such a configuration, the terminals 2709, 2720are connected to a load rather than a power source in order to providepower to a load. The heat transfer members 2708, 2707 provide thermalpower in the form of a temperature gradient. The temperature gradientbetween the first heat transfer member 2708 and second heat transfermember 2707 causes the thermoelectric system 2700 to generate a currentat terminals 2709, 2720, which in turn would connect to a load or apower storage system. Thus, the system 2700 could operate as a powergenerator. The other configurations depicted in this description couldalso be coupled in similar manners to provide a power generation systemby applying a temperature gradient and deriving a current.

A TE heat transfer system 2800 is depicted in FIG. 28 that uses a gasworking fluid 2810, and a liquid working fluid 2806. In this embodiment,first side shunt heat exchangers 2803 are of construction depicted inFIG. 24A and FIG. 24B. The shunt heat exchangers 2803 transfer thermalpower with the gaseous working media 2810. In this embodiment, secondside shunts heat exchanger 2804, 2805 transfer thermal power with liquidworking media 2806. A plurality of TE elements 2801 of oppositeconductivity types are sandwiched between second side shunts 2804, 2805and the shunt heat exchanger 2803. The second side shunt heat exchangers2804, 2805 are similarly sandwiched between TE elements 2801 ofalternating conductivity type. A current 2812, 2813 passes through thesystem 2800 as represented by the arrows 2812, 2813. In this embodiment,tubes 2814, 2815 pass the liquid working media 2806 from one shunt heatexchanger 2804, 2805 to the next one.

Operation of the TE heat transfer system 2800 is similar to that of thedescription of FIG. 24B, with one working fluid 2810 being gaseous andthe other 2806 being liquid. The benefits of thermal isolation asdescribed in U.S. Pat. No. 6,539,725 are also achieved with the designdepicted in system 2800.

FIG. 29 depicts details of a shunt heat exchanger 2900. The assemblyadvantageously has a container 2901 constructed of very good thermallyconductive material, an electrode 2902 constructed of very goodelectrically conductive material, and heat transfer fins 2905 and 2906in good thermal contact with the top and bottom surfaces of container2901. In one embodiment, the container 2901 and the electrode 2902 areconstructed of a single material, and could be unitary in construction.Advantageously, an interface 2904 between the bottom surface ofcontainer 2901 and electrode 2902 has very low electrical resistance.Fluid 2909 passes through the shunt heat exchanger 2900.

In operation, TE elements, not shown, are electrically connected to thetop and bottom portions of the electrode 2902. When suitable current isapplied through the TEs and the electrode 2902, the container 2901 andthe fins 2905, 2906 are heated or cooled. The working fluid 2909 passingthrough the shunt heat exchanger 2900 is heated or cooled by the heatexchange 2900. Advantageously, the shunt heat exchanger 2900 is ofsufficiently good electrical conductivity that it does not contributesignificantly to parasitic losses. Such losses can be made smaller byminimizing the current path length through electrode 2902, maximizingelectrical conductivity throughout the current path, and increasingelectrode 2902 cross sectional area.

The container 2901 top and bottom surfaces, and fins 2905 and 2906provide sufficient electrical conductivity in the direction of currentflow, that the solid electrode body 2902 can be reduced in crosssectional area or completely eliminated as shown in the embodiment inFIG. 4B.

A heat sink and fluid system 3000 is depicted in FIG. 30. TE elements3001 of alternating conductivity types are interspersed between fluidheat exchanges 3004, each having shunt portions 3003, and shunts 3002and 3005. Current 3006, 3007 flows through the shunt portions 3003, theshunts 3002 and 3005 and the TE elements 3001. A working fluid 3009flows as indicated by the arrow. Heat sinks 3010, 3011 are in goodthermal contact with and electrically insulated from the shunts 3002,3005. In embodiments with metallic or otherwise electrically conductiveheat sinks 3010, 3011 electrical insulation 3008, 3012 thatadvantageously has good thermal conductance confines the current flow3001, 3007 to the circuit path indicated.

When suitable current 3006, 3007 is applied, thermal power istransferred to the heat sinks 3010, 3011 and from the working fluid3009. The shunt heat transfer members 3004 are thermally isolated fromone another so that performance gains from thermal isolation areachieved with this embodiment.

An alternative shunt heat exchanger embodiment 3100 is depicted in FIG.31A. A shunt portion 3101 has electrodes 3102 for connection to TEelements (not shown) and heat transfer extensions 3108 in good thermalcontact with heat exchangers 3103, such as fins. A fluid 3107 passesthrough the heat exchangers 3103.

The shunt heat exchanger 3100 preferably has electrodes 3102 locatedgenerally centered between heat transfer extensions 3108. In thisembodiment, thermal power can flow into and out of the TE assemblies intwo directions, and thus can increase heat transfer capacity by about afactor of two per TE element in comparison to the embodiment depicted inFIG. 24A. The shunt side may have increased heat transfercharacteristics such as by incorporation heat pipes, convective heatflow, or by utilizing any other method of enhancing heat transfer.

FIG. 31B depicts a heat transfer shunt assembly 3110 with a shunt 3111,electrodes 3112 and influent fluid ports 3113, 3114, and effluent fluidports 3115, 3116. The heat transfer shunt assembly 3110 can haveincreased heat transfer capacity per TE element and more fluid transportcapacity than the system depicted in FIG. 29.

FIG. 31C depicts a shunt assembly 3120 with shunt member 3121,electrodes 3122 and heat exchange surfaces 3123, 3124. The shuntassembly 3120 can have approximately two times the heat transfercapacity per TE assembly as the embodiment depicted in FIGS. 26A and26B. However, in contrast to the usage described in FIGS. 26 A and 26B,stacks of shunt assemblies 3120 would alternate at approximately rightangles to one another and the surfaces 3123, 3124 opposite one anotherwould both be heated, for example, and the next pair of surfaces in thestack at about a right angle to the heated pair, would be cooled.Alternatively the surfaces 3123, 3214 could be at other angles such as120° and be interdispersed with shunts 2604 as depicted in FIG. 26. Anycombination of multisided shunts is part of the inventions.

It should be noted that the reduction in thermoelectric material can bequite dramatic. For example, the thermoelectric elements discussedherein may be as thin as 5 microns to 1.2 mm in one general embodiment.For superlattice and heterostructure configurations, such as could beaccomplished using the embodiments of FIGS. 31A-C, 26A-B, and 27,thermoelectric elements may be between 20 microns and 300 microns thick,more preferably from 20 microns to 200 microns, and even from 20 micronsto 100 microns. In another embodiment, the thickness of thethermoelectric elements is between 100 microns and 600 microns. Thesethicknesses for the thermoelectric elements are substantially thinnerthan conventional thermoelectric systems.

It should be noted that the configurations described do not necessarilyrequire the TE elements to be assembled into arrays or modules. For someapplications, TE elements are advantageously attached directly to heattransfer members, thereby reducing system complexity and cost. It shouldalso be noted that the features described above may be combined in anyadvantageous way without departing from the invention. In addition, itshould be noted that although the TE elements are shown in the variousfigures to appear to be of similar sizes, the TE elements could vary insize across the array or stack, the end type TE elements could be ofdifferent size and shape than the P-type TE elements, some TE elementscould be hetero structures while others could be non-hetero structure indesign.

In general, the systems described in these figures do operate in bothcooling/heating and power generation modes. Advantageously, specificchanges can be implemented to optimize performance for cooling, heatingor power generation. For example, large temperature differentials (200to 2000° F.) are desirable to achieve high-efficiency in powergeneration as is well know in the art, while small temperaturedifferentials (10 to 60° F.) are characteristic of cooling and heatingsystems. Large temperature differentials require different constructionmaterials and possibly TE modules and elements of different designdimensions and materials. Nevertheless, the basic concept remains thesame for the different modes of operation. The designs described inFIGS. 5, 8 and 9 are advantageous for power generation because theyoffer the potential to fabricate simple, rugged, low-cost designs.However, all of the above mentioned designs can have merit for specificpower generation applications and cannot be excluded.

Thermoelectric Power Generating Systems

Certain embodiments described herein provide a novel thermoelectricpower generator (TPG) system which incorporates state of the artmaterial technology with optimized thermal management. Results from anumerical model of certain embodiments described herein can simulate theoperation of the system and facilitates its design. Advancedmulti-parameter, gradient-based optimization techniques can also be usedto better understand the interactions between various design variablesand parameters in order to progress towards an optimal TPG system designin accordance with certain embodiments described herein.

In certain embodiments described herein, the system comprises a seriesof segmented thermoelectric (TE) elements (e.g., each TE elementcomprising up to three different materials). Certain embodimentsadvantageously combine thermal isolation in the direction of flow of aworking fluid with high power density TE materials integrated directlyinto the heat transfer device. Electrical current runs parallel to theheat source and sink surfaces in certain embodiments, advantageouslyallowing integration of the TE material with multiple geometric degreesof freedom. In certain embodiments in which this design attribute iscombined with a thermal isolation thermodynamic cycle, the systemadvantageously allows each TE element of the system to be optimizedsemi-independently. In certain embodiments, each P- and N-type TEelement can have a different aspect ratio selected so that the TEmaterial layers of each TE element have sufficiently high (e.g., thehighest possible or high enough to provide the desired efficiency)values for a figure of merit (ZT) in the temperature ranges applied tothe TE layers during operation. The increased design flexibility ofcertain embodiments described herein advantageously helps address TEmaterial compatibility issues associated with segmented TE elements andfluid flow that ordinarily degrade performance. Eliminating the impactof thermal expansion mismatch while still maintaining excellent thermaland electrical contacts is also advantageously achieved by certainembodiments described herein. Additional design considerations,including electrical and thermal connector design and minimizinginterfacial resistances, are also selected in certain embodimentsdescribed herein to optimize the design of the TE system. The system ofcertain embodiments is suitable for both waste heat recovery and primarypower applications.

The potential of using thermoelectrics to generate power usefully hasincreased significantly in recent years. Advancements in new highertemperature materials with figures of merit (ZT) substantially greaterthan unity are under development at places such as Michigan StateUniversity (see, e.g., K. F. Hsu et al., “Cubic AgPb_(m)SbTe_(2+m): BulkThermoelectric Materials with High Figure of Merit,” Science, Vol. 303,Feb. 6, 2004, pp. 818-821) and Lincoln Laboratory at MassachusettsInstitute of Technology (MIT) (see, e.g., T. C. Harman et al., “QuantumDot Superlattice Thermoelectric Materials and Devices,” Science, Vol.297, (2002), pp. 2229-2232). In addition, Jet Propulsion Laboratory(JPL) has had considerable success in developing material segmentationconcepts (see, e.g., T. Caillat et al., “Development of High EfficiencySegmented Thermoelectric Unicouples,” 20th Int'l Conf onThermoelectrics, Beijing, China, 2001, pp. 282-285).

Meanwhile, BSST, Inc. has demonstrated the benefits of thermal isolationin the direction of flow (see, e.g., L. E. Bell, “Use of ThermalIsolation to Improve Thermoelectric System Operating Efficiency,” 21stInt'l Conf on Thermoelectrics, Long Beach, Calif., 2002, pp. 477-487;and R. W. Diller et al., “Experimental Results Confirming ImprovedPerformance of Systems Using Thermal Isolation,” 21st Int'l Conf onThermoelectrics, Long Beach, Calif., 2002, pp. 548-550). These benefitscan include improved HVAC coefficients of performance (COP), as well ashigh power density designs that require about ⅙^(th) the TE materialusage of conventional TE-based power generator designs (see, e.g., L. E.Bell, “High Power Density Thermoelectric Systems,” 23 rd Int'l Conf onThermoelectrics, Adelaide, Australia, 2004).

Certain embodiments described herein build upon these developments andutilize additional design innovations to further increase the amount ofpower that can be extracted from a heat source using thermoelectrics.Certain embodiments are advantageously combined with high power densityconcepts (see, e.g., L. E. Bell, “Alternate Thermoelectric ThermodynamicCycles with Improved Power Generation Efficiencies,” 22 nd Int'l Conf onThermoelectrics, Hérault, France, 2003).

FIG. 32 schematically illustrates a portion of an example thermoelectricsystem 3200 in accordance with certain embodiments described herein. Incertain embodiments, the configuration schematically illustrated by FIG.32 advantageously provides various benefits, as discussed more fullybelow. In certain embodiments, the configuration schematicallyillustrated by FIG. 32 more readily accommodates TE elements ofdifferent thicknesses, areas, and thermal expansion coefficients. Thisconfiguration also accommodates the use of high power density materials,TE elements sized to provide high power density operation, and thermalisolation in the direction of working fluid flow.

The thermoelectric system 3200 comprises a first thermoelectric element3210 comprising a first plurality of segments 3212 in electricalcommunication with one another. The thermoelectric system 3200 furthercomprises a second thermoelectric element 3220 comprising a secondplurality of segments 3222 in electrical communication with one another.The thermoelectric system 3200 further comprises a heat transfer device3230 comprising at least a first portion 3232 and a second portion 3234.The first portion 3232 is sandwiched between the first thermoelectricelement 3210 and the second thermoelectric element 3220. The secondportion 3234 projects away from the first portion 3232 and is configuredto be in thermal communication with a working medium (not shown).

In certain embodiments, at least some of the first plurality of segments3212 are in series electrical communication with one another and atleast some of the second plurality of segments 3222 are in serieselectrical communication with one another. In certain embodiments, atleast some of the first plurality of segments 3212 are inseries/parallel electrical communication with one another and at leastsome of the second plurality of segments 3222 are in series/parallelelectrical communication with one another.

FIG. 32 schematically illustrates an example stack comprising three heattransfer devices 3230 separated by the first TE element 3210 and thesecond TE element 3220. Certain other embodiments comprise at least onestack comprising a plurality of TE elements (alternating P-type andN-type TE elements) and heat transfer devices, with the heat transferdevices sandwiched between at least two TE elements of the plurality ofTE elements.

The heat transfer devices 3230 of certain embodiments provide anelectrical path from the first TE element 3210 to the second TE element3220, thereby completing a TE p-n couple, such that current from acurrent source traverses the first TE element 3210, the heat transferdevice 3230, and the second TE element 3220 in series. In certain suchembodiments, the current traverses the first plurality of segments 3212in series and the current traverses the second plurality of segments3222 in series.

The heat transfer devices 3230 of certain embodiments also provide athermal path from the working fluid to the TE elements 3210, 3220.Electrical current flows parallel to the heat source and sink surfacesin the configuration schematically illustrated by FIG. 32, therebyallowing the integration of the TE material with multiple geometricdegrees of freedom. In certain embodiments, the heat transfer devices3230 thermally isolate at least some of the TE elements from at leastsome other of the TE elements. The plurality of heat transfer devices3230 are arranged in certain embodiments to provide thermal isolation ina direction of a working medium flow.

In certain embodiments, the second portion 3234 of the heat transferdevice 3230 is wider than the first portion 3232 of the heat transferdevice 3230 in at least one direction (e.g, in a direction generallyalong a direction of working medium movement). In certain embodiments,the second portion 3234 has a generally flat surface configured to be inthermal communication with the working medium.

In certain embodiments, the stack comprises a plurality of first heattransfer devices and a plurality of second heat transfer devices, withthe first and second heat transfer devices alternating along the stack.The first heat transfer devices project in a first direction and thesecond heat transfer devices project in a second direction differentfrom the first direction. The second direction in certain embodiments isgenerally opposite to the first direction, as schematically illustratedby FIG. 32. In certain embodiments, the first heat transfer devices areconfigured to be in thermal communication with a first working medium(e.g., a flowing first working fluid) and the second heat transferdevices are configured to be in thermal communication with a secondworking medium (e.g., a flowing second working fluid).

The heat transfer device 3230 having a first portion 3232 and a secondportion 3234 projecting away from the first portion 3232 in certainembodiments provides one or more benefits over rectangular-shaped heattransfer devices. To reduce electrical resistance and weight of the heattransfer device 3230, the thickness of the first portion 3232 in thedirection of electrical current flow can advantageously be minimized.Furthermore, the dimensions of the first portion 3232 in a planegenerally perpendicular to the direction of electrical current flow canadvantageously be optimized to provide sufficient electrical and thermalconductivity to the TE elements 3210, 3220. The surface area and/or thethickness of the second portion 3234 along the direction of workingfluid flow can advantageously be increased to provide a larger thermalconduit between the heat source or heat sink and the first portion 3232of the heat transfer device 3230, thereby avoiding a large thermalresistance. It is also advantageous for the second portion 3234 to bewide in a direction generally along the stack and short in a directiongenerally perpendicular to the stack. Keeping the second portion 3234short in a direction generally perpendicular to the stack advantageouslyreduces the thermal resistance from the heat source or heat sink to thesurface of the TE element. Weight, structural stability, TE surfacearea, and temperature gradients at interfaces can each be considered indesigning the final dimensions of the heat transfer device 3230.

In certain embodiments, the first plurality of segments 3212 comprisestwo, three, four, or more different thermoelectric materials. In certainembodiments, the second plurality of segments 3222 comprises two, three,four, or more different thermoelectric materials. For example, as shownin FIG. 32, the first plurality of segments 3212 has three P-typesegments 3212 a, 3212 b, 3212 c comprising different TE materials (e.g.,p-CeFe₃RuSb₁₂, p-TAGS, and p-Bi₂Te₃, respectively) and the secondplurality of segments 3222 has three N-type segments 3222 a, 3222 b,3222 c (e.g., n-CoSb₃, n-PbTe, n-Bi₂Te₃, respectively). In FIG. 32, thefirst TE element 3210 is exposed to a horizontal temperature gradientwith the hot end at the left, and the second TE element 3220 is exposedto a horizontal temperature gradient with the hot end at the right. Asdescribed more fully below, by segmenting the TE materials, the TEelements of certain embodiments can be designed to better achieve ahigher average ZT over the temperature range at which the TE elementsare intended to operate by matching the properties of the materials ofeach TE element across the TE elements to the operating temperaturegradients or temperature profile across the TE elements.

The energy conversion efficiency of a TE element generally increasesstrongly as the average dimensionless figure of merit, ZT, of the TEelement increases. FIGS. 33A and 33B show the figures of merit (ZT) asfunctions of temperature for various P-type and N-type thermoelectricmaterials, respectively, compatible with certain embodiments describedherein. A material can have a set of one or more thermoelectricproperties which determine the efficiency of the material's performanceat a given temperature, and the figure of merit is an example parametercharacteristic of the set of one or more thermoelectric properties.

For example, for low temperatures (e.g., less than 150° C.), Bi₂Te₃ hasthe highest ZT for both P-type and N-type TE materials. For intermediatetemperatures, (e.g., 150-500° C.), TAGS is an optimal P-type material,and Zn₄Sb₃ is another option for this approximate temperature range.PbTe has a high ZT for this same approximate temperature range forN-type materials. For higher temperature ranges (e.g., 500-700° C.),skutterudite (e.g., p-CeFe₄Sb₁₂, n-CoSb₃) has a high ZT. Certainembodiments described herein utilize TE elements in which the materialsand/or material combinations provide sufficiently high (e.g., thehighest possible or high enough to provide the desired efficiency)average ZT over the temperature range of use.

As an example of TE material properties, FIG. 34 depicts the figure ofmerit, ZT, as a function of temperature for three different compositionsof lead telluride (denoted by M₁, M₂, and M₃) doped with various levelsof iodine. FIG. 34 shows that no one material has the highest ZT overthe full range of temperatures from 100° C. to 570° C. Composition M₁has the highest ZT for temperatures from about 100° C. to about 335° C.,composition M₂ has the highest ZT for temperatures from about 335° C. toabout 455° C., and composition M₃ has the highest ZT for temperaturesfrom about 455° C. to about 570° C. If TE elements are fabricated fromany single composition over the 100° C. to 570° C. temperature range,the average ZT will be substantially lower than that of an elementfabricated from all three compositions suitably configured so that eachcomposition or TE segment is subjected to temperatures in the range inwhich it has the highest ZT of the three compositions. While FIG. 34corresponds to various compositions of lead telluride doped with iodine,other TE materials and dopants are also compatible with variousembodiments described herein (see, e.g., FIGS. 33A and 33B).

In certain embodiments, one of the first and second TE elements 3210,3220 comprises P-type TE materials and the other of the first and secondTE elements 3210, 3220 comprises N-type TE materials. In certain suchembodiments, the different P-type and N-type TE materials of thesegments of the first and second TE elements 3210, 3220 are selected toprovide a sufficiently high (e.g., the highest possible or high enoughto provide the desired efficiency) average ZT for the temperature rangesover which the segments of the first and second TE elements 3210, 3220are intended to operate.

For example, the first plurality of segments 3212 comprises at least afirst TE segment and a second TE segment comprising different materials.The thermoelectric system 3200 can be configured in certain embodimentsto be operated such that the first TE segment is exposed to a firsttemperature range and the second TE segment is exposed to a secondtemperature range. The first TE segment operates more efficiently in thefirst temperature range than in the second temperature range. The secondTE segment operates more efficiently in the second temperature rangethan in the first temperature range.

Referring to the system 3200 of FIG. 32, in certain embodiments, thefirst TE element 3210 comprises three TE segments 3212 a, 3212 b, 3212 cof different materials. The system 3200 is configured to be operatedsuch that the first TE segment 3212 a is exposed to a first temperaturerange, the second TE segment 3212 b is exposed to a second temperaturerange, and the third TE segment 3212 c is exposed to a third temperaturerange. The first TE segment 3212 a operates more efficiently in thefirst temperature range than in either the second or third temperatureranges. The second TE segment 3212 b operates more efficiently in thesecond temperature range than in either the first or third temperatureranges. The third TE segment 3212 c operates more efficiently in thethird temperature range than in either the second or third temperatureranges.

Similarly, in certain embodiments, the second plurality of segments 3222comprises at least a first TE segment exposed to a first temperaturerange and a second TE segment exposed to a second temperature range, thefirst and second TE segments comprising different materials. The firstTE segment operates more efficiently in the first temperature range thanin the second temperature range. The second TE segment operates moreefficiently in the second temperature range than in the firsttemperature range. Referring to FIG. 32, in certain embodiments, thesecond TE element 3220 comprises three TE segments 3222 a, 3222 b, 3222c of different materials and is configured to be operated such that thefirst TE segment 3222 a is exposed to a first temperature range, thesecond TE segment 3222 b is exposed to a second temperature range, andthe third TE segment 3222 c is exposed to a third temperature range. Thefirst TE segment 3222 a operates more efficiently in the firsttemperature range than in either the second or third temperature ranges.The second TE segment 3222 b operates more efficiently in the secondtemperature range than in either the first or third temperature ranges.The third TE segment 3222 c operates more efficiently in the thirdtemperature range than in either the second or third temperature ranges.

In certain embodiments, various other factors may also be considered inselecting the TE materials to be used as a function of operatingtemperature, including but not limited to, thermal stability, mechanicalstability, and cost. As described more fully below, another factor indesigning TE elements compatible with certain embodiments describedherein is the impact of compatibility mismatch on optimum power outputwhen the efficiencies for different element segments occur atsignificantly different current densities (e.g., compatibility factor),(see, e.g., J. G. Snyder, “Thermoelectric Power Generation: Efficiencyand Compatibility,” Thermoelectrics Handbook, Macro to Nano, Edited byD. M. Rowe, Ph.D., D.Sc. (2006)).

Power curves for TE materials are generally parabolic with increasingcurrent. For segmented TE elements in which different TE materials areused together, the power curves of the TE elements and/or the segmentscan have their optimum power outputs occurring at significantlydifferent current densities. These differences in power curves canreduce the overall efficiency of a segmented TE element.

In certain embodiments in which the temperatures across the TE elementsdiffer (e.g., having a series of TE elements assembled in the directionof working fluid flow), the effects of such power curve compatibilityconflicts can be significant. FIG. 35 shows the power curvecompatibility conflict among three TE elements constructed in series inthe direction of flow where the hot side temperature T_(h) is decliningfrom 700 K to 500 K. A first TE element is exposed to a hot sidetemperature T_(h)=700 K, a second TE element is exposed to a hot sidetemperature T_(h)=600 K, and a third TE element is exposed to a hot sidetemperature T_(h)=500 K. The cold side temperature T_(c) for each ofthese three TE elements remains constant for this example at 300 K.Ideally, each TE element would operate at a current that produces peakpower output. However, since the three TE elements are electricallyconnected in series, they each are run using the same current. While thefirst TE element has its maximum power at 130 A, the other two TEelements have output powers that are suboptimal (e.g., less than theircorresponding maximum powers) at this current. In particular, the thirdelement operating between 500 K and 300 K has zero output power at 130A. The total peak power output for this example is 7.69 W, considerablybelow the individual peak power outputs of the individual TE elements.In other examples, the output power of the third TE element can benegative at the optimal current of the first TE element, such that thethird TE element subtracts power from the other two TE elements.

In certain embodiments, the form factors or shapes of the TE elementsare advantageously selected so that the power produced by each TEelement operates at a current which provides peak power or peakefficiency. In certain such embodiments, the aspect ratios of the TEelements are changed in the direction of flow, thereby advantageouslyreducing the effects of TE compatibility conflicts among the TEelements. For example, referring to FIG. 32, in certain embodiments, thefirst TE element 3210 has a first thickness along a first direction(e.g., the direction of current flow through the first TE element 3210)and a first cross-sectional area in a plane generally perpendicular tothe first direction. The second TE element 3220 has a second thicknessalong a second direction (e.g., the direction of current flow throughthe second TE element 3220) and a second cross-sectional area in a planegenerally perpendicular to the second direction. In certain embodiments,the second thickness is greater than the first thickness. In certainother embodiments, the first TE element 3210 has a first aspect ratioequal to the first cross-sectional area divided by the first thickness,and the second TE element 3220 has a second aspect ratio equal to thesecond cross-sectional area divided by the second thickness. In certainsuch embodiments, the second aspect ratio is different than the firstaspect ratio. For example, the first aspect ratio and the second aspectratio can be selected such that under operating conditions the first TEelement 3210 and the second TE element 3220 both operate at optimalefficiency.

FIG. 36 shows the power curves among three TE elements with varyingaspect ratios for an example device in accordance with certainembodiments described herein. As for FIG. 35, the three TE elements ofFIG. 36 are constructed in series in the direction of flow where the hotside temperatures T_(h) of the three TE elements are 700 K, 600 K, and500 K, respectively, and the cold side temperature T_(c) for each TEelement is 300 K. Each of the three TE elements of FIG. 35 had athickness of 1 mm. For FIG. 36, the first TE element had a thickness of1 mm, the second TE element had a thickness of 0.77 mm, and the third TEelement had a thickness of 0.5 mm. As shown by FIG. 36, changing theaspect ratio of the second and third TE elements in the seriesadvantageously aligns the currents at which the TE elements achievetheir maximum power, thereby increasing the total power density of thedevice. The total peak power output for the example device of FIG. 36 is11.51 W, a 50% improvement over the power output of the example deviceof FIG. 35.

FIG. 37 schematically depicts a pair of segmented TE elements in aconventional configuration 3700. The conventional configuration 3700 hasa first TE segmented TE element 3710 and a second TE element 3720. Eachof the first and second TE elements 3710, 3720 are coupled to onesurface of an electrically conductive and thermally conductive coupler3730. The first TE element 3710 has three P-type segments 3712 a, 3712b, 3712 c (e.g., p-CeFe₃RuSb₁₂, p-TAGS, and p-Bi₂Te₃, respectively) andthe second TE element 3720 has three N-type segments 3722 a, 3722 b,3722 c (e.g., n-CoSb₃, n-PbTe, n-Bi₂Te₃, respectively). In FIG. 37, thetemperature gradient is vertical with the hot end at the top.

In conventional TE configurations, as shown schematically in FIG. 37,the TE elements 3710, 3720 are integrated in a TE module such that eachTE element 3710, 3720 has the same thickness along the direction ofcurrent flow. Such conventional TE configurations 3700 do not lendthemselves easily to the use of TE elements having differentthicknesses, areas, or aspect ratios. In addition, such conventional TEconfigurations 3700 are difficult to control if the TE elements are ofthe same thickness but have different thermal expansion coefficients ina direction generally parallel to the direction of current flow throughthe TE element. Such thermal expansion mismatches can be particularlyproblematic in power generation systems in which operating temperaturescan be quite high. In contrast, in certain embodiments described herein(e.g., the configuration schematically illustrated by FIG. 32) in whichthe heat transfer devices 3230 are sandwiched between two TE elements3210, 3220, TE elements of different thicknesses, areas, and/or aspectratios are advantageously easily incorporated in the system.Furthermore, such configurations advantageously reduce or avoid problemsassociated with differing thermal expansion coefficients among the TEelements.

In certain embodiments in which the temperatures across the segments ofa TE element differ from one another (e.g., having the TE elementbetween a heat source and a heat sink), the effects of power curvecompatibility conflicts among the segments on the overall power outputand/or efficiency can be significant. In certain embodiments, suchincompatibilities between the segments can be at least partiallycounteracted by advantageously selecting a different aspect ratio (e.g.,cross-sectional area divided by the thickness) for each segment of theTE element. In certain embodiments, the aspect ratio is changed amongthe different segments of a TE element by maintaining a substantiallyuniform cross-sectional area and varying the thickness of each segmentto better match the current for optimal power output. In certain otherembodiments, the aspect ratios of the segments can be optimized byconstructing a segmented TE element with non-uniform cross-sectionalareas among the segments.

For example, referring to the example system 3200 of FIG. 32, in certainembodiments, each segment of the first plurality of segments 3212 has athickness along the direction of current flow through the first TEelement 3210 different from the thicknesses of the other segments of thefirst plurality of segments 3212. In certain such embodiments, eachsegment of the second plurality of segments 3222 has a thickness alongthe direction of current flow through the second TE element 3220different from the thicknesses of the other segments of the secondplurality of segments 3222. In certain embodiments, each segment of thefirst plurality of segments 3212 has an aspect ratio equal to athickness of the segment divided by a cross-sectional area of thesegment, and the aspect ratios of the segments of the first plurality ofsegments 3212 are different from one another. In certain suchembodiments, each segment of the second plurality of segments 3222 hasan aspect ratio equal to a thickness of the segment divided by across-sectional area of the segment, and the aspect ratios of thesegments of the second plurality of segments 3222 are different from oneanother. The aspect ratios of the segments of the first plurality ofsegments 3212 and the aspect ratios of the segments of the secondplurality of segments 3222 are selected in certain embodiments such thatunder operating conditions the first thermoelectric element 3210 and thesecond thermoelectric element 3220 both operate at optimal efficiency.

FIG. 38 shows the average efficiencies for three differentconfigurations simulated using a model calculation. The model simulatedthe performance of three different configurations of a stack of three TEelements and two heat transfer devices which thermally isolated the TEelements from one another along the direction of flow of a workingfluid, in accordance with certain embodiments described herein. The hotside temperatures of the TE elements varied from 700° C. to 300° C.while the cold side temperatures varied from 100° C. to 150° C., suchthat the temperature differences across the three TE elements were 550°C., 375° C., and 200° C., respectively.

In a first configuration (labelled “uniform non-segmentation & aspectratio” in FIG. 38), all three of the TE elements were made of a singlematerial (non-segmented), the material of each of the TE elements wasthe same as the others (with two doped to be N-type, and one doped to beP-type), and each TE element had the same aspect ratio. In a secondconfiguration (labelled “uniform segmentation & aspect ratio” in FIG.38), the TE elements were segmented to better take advantage of theoptimal ZT over each TE element's operating temperature range, the twoN-type TE elements were segmented in the same way, and all three TEelements had the same aspect ratio. In a third configuration (labelled“non-uniform segmentation & aspect ratio” in FIG. 38), the two N-type TEelements were segmented differently for each TE element's particulartemperature range, and the aspect ratio of each TE element variedadvantageously. In all three configurations, the TE elements wereconnected electrically in series such that the same current traversedeach TE element.

FIG. 38 shows that the TE material compatibility as well as TE elementcompatibility in the direction of flow causes the first configuration tobe 35% less efficient than the second configuration, while the secondconfiguration is 15% less efficient than the third configuration. Thesedifferences become more dramatic with more thermally isolated TEelements along the direction of flow. FIG. 38 illustrates the advantagesprovided by certain embodiments described herein which combinenon-uniform segmentation of the TE elements with optimization of theaspect ratios in the direction of flow.

In certain embodiments utilizing the configuration schematicallyillustrated by FIG. 32, each TE element can be optimizedsemi-independently of the other TE elements. For example, each P-typeand N-type TE element can have a different cross-sectional area and/orthickness with each segment of each TE element having a sufficientlyhigh ZT at each particular temperature range.

In certain embodiments, thermal expansion mismatch can advantageously beconsidered when selecting a material to join the heat transfer devicesand the TE elements to assemble a thermoelectric system for hightemperature power generation applications. Certain embodiments describedherein utilize non-rigid connections to at least partially relievethermal stresses due to thermal expansion mismatch between differentportions of the thermoelectric system. In certain embodiments, thenon-rigid connection advantageously prevents complications caused bythermal expansion mismatch between the heat transfer device and the TEelement. In certain embodiments, the non-rigid connection alsoadvantageously protects against the mismatch of expansion between thehot and cold sides of segmented TE elements.

In certain such embodiments, the thermoelectric system comprises one ormore liquid metal joints between at least one TE element and at leastone neighboring heat transfer devices to provide at least one non-rigidthermally and electrically conductive connections. For example, thethermoelectric system 3200 schematically illustrated by FIG. 32 cancomprise a first liquid metal joint in thermal and electricalcommunication with the first TE element 3210 and the heat transferdevice 3230, and a second liquid metal joint in thermal and electricalcommunication with the heat transfer device 3230 and the second TEelement 3220. This joint can either be liquid at room temperature or canmelt at a temperature lower than the temperature applied to the jointduring operation of the system. For example, standard SnPb solder can beused on a hot side of a TE element with operating temperatures that farexceed the solder's melting point.

Utilizing one or more liquid metal joints can introduce severalcomplications in the fabrication of the thermoelectric system. Incertain embodiments, additional structure may be used to providestructural integrity. This additional structure can advantageously bethermally insulating. In certain embodiments, the at least one stack isunder compression generally along the stack. In addition, some level ofcontrol can advantageously be provided in certain embodiments to preventthe liquid metal from flowing out of the joint area and shorting out thedevice. In certain embodiments, proper material combinations canadvantageously be used to prevent accelerated corrosion or undesiredalloying (e.g., resulting in brittleness of the bonds or reduced thermalor electrical conductivity) at the interface due to maintaining a liquidmetal at high temperatures.

In certain embodiments, non-rigid joints are advantageously used toreduce or eliminate the buildup of thermal stresses at the interfacesbetween the heat transfer devices and the heat sources or heat sinks.The second portion 3232 of the heat transfer device 3230 can result inthermal stress buildup in the x-plane between heat transfer devices,particularly on the hot side. Thermal expansion coefficients for the TEmaterials between the heat transfer devices can be difficult to match tothe thermal expansion coefficient of the heat source. Thus, in certainembodiments, the heat transfer devices are advantageously connected tothe heat source using a liquid metal. In certain embodiments, the liquidmetal at this interface is constrained so as to make avoid creating anelectrical short between two heat transfer devices. The liquid metal canbe advantageously contained to the immediate joint area. In certainembodiments, the heat transfer devices are joined to the heat sink(e.g., less than 400° C.) using thermal grease. In certain embodiments,the thermoelectric system is placed in compression in order to holdeverything in place without the use of a rigid structural connector.This compression in certain embodiments can also improve thermal contactin the y-plane and thermal and electrical contact in the x-plane.

In certain embodiments, molybdenum can be used to provide a thermallyand electrically conductive joint. For example, the thermoelectricsystem 3200 schematically illustrated by FIG. 32 can comprises amolybdenum layer between the first TE element 3210 and the heat transferdevice 3230 and a molybdenum layer between the heat transfer device 3230and the second TE element 3220. Molybdenum, despite having one third theelectrical and thermal conductivity of copper and being slightly moredense than copper, can be used as the connector material for the hotside. As a refractory metal, molybdenum does not corrode as easily ascopper with many liquid metals, and it has a very low thermal expansioncoefficient as compared to copper. These attributes are advantageouswhen joining the heat transfer device to an electrical isolation layer(e.g., a ceramic with very low thermal expansion coefficient). Incertain embodiments, a high thermal and non-electrical conductivityaluminum nitride can be used for the barrier between the heat transferdevice and the heat source. Electrical isolation is advantageously usedbetween the heat transfer devices and the heat sources or heat sinks toprevent electrical current from flowing through the working fluids.Electrical current flowing in some working fluids can greatly acceleratethe fouling of the heat transfer device. Suitable ceramic layers oncopper will crack at high operating temperatures due to the largethermal expansion mismatch. However, in certain embodiments, molybdenumcan provide a good compromise. Molybdenum does have its complications.For example, molybdenum is not wet very well by many liquid metals,thereby increasing the electrical and thermal interfacial resistance. Toimprove molybdenum wettability, in certain embodiments, the molybdenumcan be plated with a thin layer of nickel followed by a gold flash, andthe outer metallization of the TE elements can be a similar nickel/goldcombination.

In certain embodiments, a method of fabricating a thermoelectric systemis provided. The method comprises providing a plurality ofthermoelectric elements, with at least some of the thermoelectricelements comprising a plurality of segments. The method furthercomprises providing a plurality of heat transfer devices, with at leastsome of the heat transfer devices comprising at least a first portionand a second portion. The method further comprises assembling theplurality of thermoelectric elements and the plurality of heat transferdevices to form at least one stack of alternating thermoelectricelements and heat transfer devices. The first portions of the heattransfer devices are sandwiched between at least two neighboringthermoelectric elements. The second portions of the heat transferdevices project away from the stack and are configured to be in thermalcommunication with a working medium.

In certain embodiments, assembling the plurality of thermoelectricelements and the plurality of heat transfer devices comprises placing aliquid metal joint between at least one thermoelectric element and atleast one neighboring heat transfer device to place the at least onethermoelectric element and the at least one neighboring heat transferdevice in thermal communication and in series electrical communicationwith one another.

In certain embodiments, the at least some of the thermoelectric elementshave aspect ratios, with the aspect ratio of a thermoelectric elementequal to a cross-sectional area of the thermoelectric element in a planegenerally perpendicular to the stack divided by a thickness of thethermoelectric element in a direction generally parallel to the stack.The aspect ratios for the at least some of the thermoelectric elementsvary from one another along the stack. In certain such embodiments, theaspect ratios are selected such that under operating conditions the atleast some of the thermoelectric elements operate at optimal efficiency.

In certain embodiments, each segment of the plurality of segments of athermoelectric element has an aspect ratio equal to a cross-sectionalarea of the segment in a plane generally perpendicular to the stackdivided by a thickness of the segment in a direction generally parallelto the stack. The aspect ratios of the segments can vary from oneanother along the thermoelectric element. In certain such embodiments,the aspect ratios are selected such that under operating conditions thesegments of the plurality of segments operate at optimal efficiency.

Certain embodiments described herein have been modeled using aMATLAB-based numerical, steady-state model based in part on previouswork (see, e.g., D. T. Crane, “Optimizing Thermoelectric Waste HeatRecovery from an Automotive Cooling System”, PhD Dissertation,University of Maryland, College Park, 2003). The model usedsimultaneously solved, non-linear, energy balance equations whichsimulate certain embodiments of the high power density TE assembliesdiscussed herein. The principles used in the current model were alsoused in a previous TE model developed by BSST (see, D. T. Crane,“Modeling High-Power Density Thermoelectric Assemblies Which Use ThermalIsolation,” 23 rd International Conference on Thermoelectrics, Adelaide,AU. 2004. This previous TE model was validated for heating and coolingapplications and was previously shown to be accurate to within 7% forfour different outputs. The average error for each of these simulatedvalues was less than 3%.

The TE segmented material information of certain embodiments wasincorporated into the model using algorithms and equations described byG. J. Snyder, “Thermoelectric Power Generation: Efficiency andCompatibility,” in Thermoelectrics Handbook Macro To Nano, Rowe, D. M.,Editor. CRC Press (Boca Raton, Fla., 2006), pp. 9-1-9-26). The model canbe used to automatically solve for the optimal TE segmentation for agiven set of hot and cold side temperatures. The thicknesses of thematerial segments and the material layers themselves can be allowed tovary to determine optimal performance for a given electrical loadresistance. The model can also solve for off-nominal solutions by fixingthe material layer thicknesses.

Using the model, various design variables in certain embodiments wereidentified and varied to analyze the trade-offs involved in improvingefficiency. Advanced multi-parameter, gradient-based optimizationstudies were used to better understand the interactions between variousdesign variables, parameters, and constraints and to develop an optimalthermoelectric power generation (TPG) design in accordance with certainembodiments described herein.

Optimization analysis of certain embodiments can also include parametricanalyses. FIG. 39 shows an example of such an analysis of athermoelectric system where the parameter being varied is the TEthickness. FIG. 39 shows the tradeoffs between high power density andhigh efficiency. Changing the TE thickness has a more dramatic effect onTE power density than on total heat exchanger power density, whichremains relatively unchanged. Using such a parametric analysis, certainembodiments described herein can be designed for a particularapplication. For example, in automotive waste heat recoveryapplications, it is very desirable to have as high an efficiency aspossible, but having a high power density is also desirable.

Initial modeling for certain embodiments described herein was completed,and building and testing some fractional prototype devices was alsoperformed to fully validate the model. The model can then be used moreextensively to complete the analysis of particular device designs inaccordance with certain embodiments described herein.

FIG. 40 shows an example prototype system built using six Bi₂Te₃ TEelements sandwiched between seven copper heat transfer devices. TheBi₂Te₃ TE elements were used because the tests were conducted at lowertemperatures with materials that have well-defined properties. Thesetests were conducted to better isolate problem areas in the integrationof TE materials into a system. Copper heat transfer devices were used onthe hot side of this system because the temperatures were lower thanthose seen in higher temperature applications. In certain embodimentsfor use at higher temperatures, molybdenum heat transfer devices canreplace the copper heat transfer devices on the hot side of the system.

For assembly simplicity, the system shown in FIG. 40 used rectangular TEheat transfer devices rather than heat transfer devices having a secondportion wider than the first portion as used in certain embodimentsdescribed herein. The copper heat transfer devices were placed on analuminum tube, which served as the heat sink for the system. Thealuminum tube was anodized to provide electrical isolation from thecopper heat transfer devices. A layer of thermal grease covered theanodized layer to help minimize thermal resistance. Two 100 W cartridgeheaters provided the heat source for the system, and were enclosed in ananodized aluminum housing. For assembly simplicity, thermal grease wasused at low temperatures (e.g., less than 400° C.) as the thermalinterface material between the aluminum housing and the heat transferdevices. For certain embodiments to be used at higher temperatures,liquid metal can be used instead. A prototype test fixture wasconstructed to test the fractional build described above.

FIG. 41 is a graph showing power generation curves for the sixindividual Bi₂Te₃ elements of FIG. 40. Using the knowntemperature-dependent Seebeck coefficient for Bi₂Te₃, the temperaturedifference across the TE element could be derived from the measured opencircuit voltage and compared to the temperature difference measured withthermocouples. The difference between hot and cold side temperaturemeasurements and those calculated at zero current were then applied asan offset for the temperatures at all currents.

Using the known temperature-dependent electrical resistivity propertyfor Bi₂Te₃, the electrical resistivity followed by the electricalresistance was calculated at the new adjusted temperatures. The bulkjoint resistance could be calculated by subtracting the measured voltageat a particular current from the calculated open circuit voltage at themeasured temperature difference at the particular current, and dividingby the measured current. This bulk resistance included the resistance ofthe TE element as well as the contact resistances created by the solderand the TE element plating. The resistance of the copper heat transferdevices was considered negligible when compared to the resistances ofthe TE element and the interfacial resistances. Subtracting thecalculated TE element resistance from this bulk joint resistancerevealed the contact resistance for the joints on both sides of the TEelement. With the known surface area of the TE element, the electricalinterfacial resistivity could then be calculated for each TE element.

Using these calculated temperature-independent electrical interfacialresistivities along with the current-independent temperature offsets,the power generation curves shown in FIG. 41 were calculated usingstandard thermoelectric equations. The dotted lines in FIG. 41 representthe calculated power curves compared to the measured power curvesrepresented by the solid lines. It can be seen from FIG. 41 that thismethod of estimation can be very accurate for all six elements.

With matched power curves, the hot and cold side surface temperatures aswell as the electrical interfacial resistivities could be accepted andanalyzed for their absolute values and their consistency between TEelements. The estimated electrical interfacial resistivities can becompared to those described in the literature (see, e.g., G. S. Nolas etal., “Thermoelectrics—Basic Principles and New Materials Developments,”Springer-Verlag (Berlin Heidelberg, 2001)). FIG. 41 shows that all sixTE elements had interfacial resistivities less than 10 μΩcm², which canbe considered to be a reasonable value. These tests were conducted tosee how low this interfacial resistance could be and how consistently itcould be achieved across each TE element. In the test shown in FIG. 41,the four middle TE elements have relatively low and consistentinterfacial resistivities. The two end TE elements have two to threetimes the interfacial resistance of the inner TE elements. This effectcould be due to additional stress put on these TE elements due to beingon the ends of the assembly.

The results of these tests for Bi₂Te₃ elements can be carried over tothe tests and device design for higher temperature materials inaccordance with certain embodiments described herein. Interfacialresistivities and temperature drops across the interfaces can be similarfor these TE elements.

FIG. 42 shows experimental results for initial testing of segmented TEelements. Two N-type TE elements having the dimensions and materialslisted in FIG. 42 were tested. The same prototype fixture and systemconfiguration to those described above with regard to FIGS. 40 and 41were used for this test as well. Temperatures of the cold side bath,heater settings, and the measured TE surface temperatures are alsolisted in FIG. 42. The curves of FIG. 42 are different due to thedesigned difference in the TE element and layer thicknesses as well asthe slightly different temperature drops. FIG. 42 shows that the amountof power recovered increases with increasing hot side temperature.Optimal current also increases slightly with increasing temperature.Element 1 produced the maximum power at 8 A at a hot side temperature of172° C. and at 11.3 A at 366° C. Further testing and analysis on theseand other similar P- and N-type segmented elements can be used todetermine the same level of predictability as the tests on the Bi₂Te₃.

Certain embodiments described herein significantly improve the abilityof thermoelectric power generation to achieve higher power outputs andefficiencies. Certain embodiments described herein address the issues ofTE compatibility mismatch not only within an element, but also withrespect to elements in the direction of flow to greatly improve TEsystem performance for many applications. In certain embodiments, theuse of heat transfer devices with a second portion extending from afirst portion sandwiched between two TE elements with the second portionwider than the first portion advantageously helps incorporate thermalisolation in the direction of flow and non-uniform high power densityelements in a usable system. Certain such embodiments advantageouslyreduce the effects of thermal expansion mismatch, which would otherwisemake it more difficult to construct a TE device with elements ofdiffering thickness. Certain embodiments described herein use liquidmetal joints to reduce the effects of thermal expansion mismatch toadvantageously aid in the construction of a system that will holdtogether under high operating temperatures.

The advanced modeling and optimization techniques described hereinadvantageously help optimize the design concepts of certain embodimentsto progress towards maximizing the performance of a TPG system.Prototype builds and tests also help validate the design concepts andthe models. A full-scale TPG system in accordance with certainembodiments described herein can be used to recover waste heat fromautomotive exhaust, for primary power applications, or many otherdifferent waste heat recovery applications, including those associatedwith integrating a TE system into a fuel cell.

It should also be noted that the disclosures in this patent presentdesigns, configurations and applications of this invention. While thediscussion above is analyzed in terms of the properties in cooling,similar results hold for heating and power generation, and lead tosimilar conclusions. Some systems, in particular those of the thermionicand heterostructure type, may be intrinsically of high power density, inwhich case this invention can be more suitable to accommodate theproperties and possible high power densities of such systems.

Although several examples have been illustrated, and discussed above,the descriptions are merely illustrative of broad concepts of theinventions, which are set forth in the attached claims. In the claims,all terms are attributed to their ordinary and accustomed meaning andthe description above does not restrict the terms to any special orspecifically defined means unless specifically articulated.

1. A thermoelectric system comprising: a first thermoelectric elementcomprising a first plurality of segments in electrical communicationwith one another; a second thermoelectric element comprising a secondplurality of segments in electrical communication with one another; anda heat transfer device comprising at least a first portion and a secondportion, the first portion sandwiched between the first thermoelectricelement and the second thermoelectric element, the second portionprojecting away from the first portion and configured to be in thermalcommunication with a working medium.
 2. The thermoelectric system ofclaim 1, wherein the first thermoelectric element has a first thicknessalong a first direction, the second thermoelectric element has a secondthickness along a second direction, and the second thickness is greaterthan the first thickness.
 3. The thermoelectric system of claim 1,wherein the first thermoelectric element has a first thickness along afirst direction, a first cross-sectional area in a plane generallyperpendicular to the first direction, and a first aspect ratio equal tothe first cross-sectional area divided by the first thickness, thesecond thermoelectric element has a second thickness along a seconddirection, a second cross-sectional area in a plane generallyperpendicular to the second direction, and a second aspect ratio equalto the second cross-sectional area divided by the second thickness,wherein the second aspect ratio is different than the first aspectratio.
 4. The thermoelectric system of claim 3, wherein the first aspectratio and the second aspect ratio are selected such that under operatingconditions the first thermoelectric element and the secondthermoelectric element both operate at optimal efficiency.
 5. Thethermoelectric system of claim 1, wherein each segment of the firstplurality of segments has a thickness different from the thickness ofother segments of the first plurality of segments.
 6. The thermoelectricsystem of claim 5, wherein each segment of the second plurality ofsegments has a thickness different from the thickness of other segmentsof the second plurality of segments.
 7. The thermoelectric system ofclaim 1, wherein each segment of the first plurality of segments has anaspect ratio equal to a thickness of the segment divided by across-sectional area of the segment, wherein the aspect ratios of thesegments of the first plurality of segments are different from oneanother.
 8. The thermoelectric system of claim 7, wherein each segmentof the second plurality of segments has an aspect ratio equal to athickness of the segment divided by a cross-sectional area of thesegment, wherein the aspect ratios of the segments of the segmentplurality of segments are different from one another.
 9. Thethermoelectric system of claim 8, wherein the aspect ratios of thesegments of the first plurality of segments and the aspect ratios of thesegments of the second plurality of segments are selected such thatunder operating conditions the first thermoelectric element and thesecond thermoelectric element both operate at optimal efficiency. 10.The thermoelectric system of claim 1, wherein the first plurality ofsegments comprises at least a first thermoelectric segment and a secondthermoelectric segment, the first and second thermoelectric segmentscomprising different materials.
 11. The thermoelectric system of claim10, wherein the thermoelectric system is configured to be operated suchthat the first thermoelectric segment is exposed to a first temperaturerange and the second thermoelectric segment is exposed to a secondtemperature range, wherein the first thermoelectric segment operatesmore efficiently in the first temperature range than in the secondtemperature range and the second thermoelectric segment operates moreefficiently in the second temperature range than in the firsttemperature range.
 12. The thermoelectric system of claim 11, whereinthe first plurality of segments comprises a third thermoelectricsegment, wherein the thermoelectric system is configured to be operatedsuch that the third thermoelectric segment is exposed to a thirdtemperature range, wherein the third thermoelectric segment operatesmore efficiently in the third temperature range than in either thesecond temperature range or the first temperature range.
 13. Thethermoelectric system of claim 10, wherein the second plurality ofsegments comprises at least a first thermoelectric segment and a secondthermoelectric segment, the first and second thermoelectric segmentscomprising different materials.
 14. The thermoelectric system of claim13, wherein the thermoelectric system is configured to be operated suchthat the first thermoelectric segment is exposed to a first temperaturerange and the second thermoelectric segment is exposed to a secondtemperature range, wherein the first thermoelectric segment operatesmore efficiently in the first temperature range than in the secondtemperature range and the second thermoelectric segment operates moreefficiently in the second temperature range than in the firsttemperature range.
 15. The thermoelectric system of claim 14, whereinthe first plurality of segments comprises a third thermoelectricsegment, wherein the thermoelectric system is configured to be operatedsuch that the third thermoelectric segment is exposed to a thirdtemperature range, wherein the third thermoelectric segment operatesmore efficiently in the third temperature range than in either thesecond temperature range or the first temperature range.
 16. Thethermoelectric system of claim 1, wherein at least some of the firstplurality of segments are in series electrical communication with oneanother and at least some of the second plurality of segments are inseries electrical communication with one another.
 17. The thermoelectricsystem of claim 1, wherein at least some of the first plurality ofsegments are in series/parallel electrical communication with oneanother and at least some of the second plurality of segments are inseries/parallel electrical communication with one another.
 18. Thethermoelectric system of claim 1, wherein the second portion is widerthan the first portion in at least one direction.
 19. The thermoelectricsystem of claim 18, wherein the second portion is wider than the firstportion in a direction generally along a direction of working mediummovement.
 20. The thermoelectric system of claim 18, wherein the secondportion has a generally flat surface configured to be in thermalcommunication with the working medium.
 21. The thermoelectric system ofclaim 1, further comprising a current source in electrical communicationwith the first thermoelectric element, the heat transfer device, and thesecond thermoelectric element, such that a current from the currentsource traverses the first thermoelectric element, the heat transferdevice, and the second thermoelectric element in series.
 22. Thethermoelectric system of claim 21, wherein the current traverses thefirst plurality of segments in series and the current traverses thesecond plurality of segments in series.
 23. The thermoelectric system ofclaim 1, further comprising a first liquid metal joint in thermal andelectrical communication with the first thermoelectric element and theheat transfer device and a second liquid metal joint in thermal andelectrical communication with the heat transfer device and the secondthermoelectric element.
 24. The thermoelectric system of claim 23,wherein the first liquid metal joint and the second liquid metal jointcomprise SnPb solder.
 25. The thermoelectric system of claim 1, furthercomprising a molybdenum layer between the first thermoelectric elementand the heat transfer device and a second molybdenum layer between theheat transfer device and the second thermoelectric element.
 26. Athermoelectric system comprising: a plurality of thermoelectricelements, at least some of the thermoelectric elements comprising aplurality of segments; and a plurality of heat transfer devices, atleast some of the heat transfer devices comprising at least a firstportion and a second portion, the first portion sandwiched between atleast two thermoelectric elements of the plurality of thermoelectricelements so as to form at least one stack of thermoelectric elements andheat transfer devices, the second portion projecting away from the stackand configured to be in thermal communication with a working medium. 27.The thermoelectric system of claim 26, wherein the heat transfer devicesthermally isolate at least some of the thermoelectric elements from atleast some other of the thermoelectric elements.
 28. The thermoelectricsystem of claim 26, wherein the plurality of heat transfer devices arearranged to provide thermal isolation in a direction of a working mediumflow.
 29. The thermoelectric system of claim 26, wherein thethermoelectric elements comprise alternating P-type and N-typethermoelectric elements.
 30. The thermoelectric system of claim 26,wherein the plurality of heat transfer devices comprise a plurality offirst heat transfer devices and a plurality of second heat transferdevices, the stack comprising alternating first and second heat transferdevices, wherein the second portions of the first heat transfer devicesproject in a first direction and the second portions of the second heattransfer devices project in a second direction, the second directiondifferent from the first direction.
 31. The thermoelectric system ofclaim 30, wherein the second direction is generally opposite to thefirst direction.
 32. The thermoelectric system of claim 30, wherein thefirst heat transfer devices are configured to be in thermalcommunication with a first working medium and the second heat transferdevices are configured to be in thermal communication with a secondworking medium.
 33. The thermoelectric system of claim 26, furthercomprising at least one liquid metal joint between at least onethermoelectric element and at least one neighboring heat transferdevice, and the at least one stack is under compression generally alongthe stack.
 34. The thermoelectric system of claim 26, wherein at leastsome of the thermoelectric elements are sized to provide high powerdensity operation.
 35. The thermoelectric system of claim 26, furthercomprising a current source in electrical communication with the stack,such that a current from the current source traverses the thermoelectricelements and the heat transfer devices in series.
 36. The thermoelectricsystem of claim 35, wherein the current traverses the plurality ofsegments of the at least some of the thermoelectric elements in series.37. A method of fabricating a thermoelectric system, the methodcomprising: providing a plurality of thermoelectric elements, at leastsome of the thermoelectric elements comprising a plurality of segments;providing a plurality of heat transfer devices, at least some of theheat transfer devices comprising at least a first portion and a secondportion; and assembling the plurality of thermoelectric elements and theplurality of heat transfer devices to form at least one stack ofalternating thermoelectric elements and heat transfer devices, whereinthe first portions of the heat transfer devices are sandwiched betweenat least two neighboring thermoelectric elements, the second portions ofthe heat transfer devices projecting away from the stack and configuredto be in thermal communication with a working medium.
 38. The method ofclaim 37, wherein assembling the plurality of thermoelectric elementsand the plurality of heat transfer devices comprises placing a liquidmetal joint between at least one thermoelectric element and at least oneneighboring heat transfer device to place the at least onethermoelectric element and the at least one neighboring heat transferdevice in thermal communication and in series electrical communicationwith one another.
 39. The method of claim 37, wherein the at least someof the thermoelectric elements have aspect ratios, the aspect ratio of athermoelectric element equal to a cross-sectional area of thethermoelectric element in a plane generally perpendicular to the stackdivided by a thickness of the thermoelectric element in a directiongenerally parallel to the stack, wherein the aspect ratios for the atleast some of the thermoelectric elements vary from one another alongthe stack.
 40. The method of claim 39, wherein the aspect ratios areselected such that under operating conditions the at least some of thethermoelectric elements operate at optimal efficiency.
 41. The method ofclaim 37, wherein each segment of the plurality of segments of athermoelectric element has an aspect ratio equal to a cross-sectionalarea of the segment in a plane generally perpendicular to the stackdivided by a thickness of the segment in a direction generally parallelto the stack, wherein the aspect ratios of the segments vary from oneanother along the thermoelectric element.
 42. The method of claim 41,wherein the aspect ratios are selected such that under operatingconditions the segments of the plurality of segments operate at optimalefficiency.